Vectorial translation

The requisites for the supramolecular antenna sensitizers are (1) an efficient antenna effect, vectorially translating absorbed energy toward a molecular component and (2) the capability of the molecular component bound to the semiconductor surface to inject electrons into the semiconductor from its excited state. Antenna sensitizers can increase the fraction of light harvested by a sensitized semiconductor surface. Two simple prototypes, following the branched or onedimensional design, are shown schematically in Fig. 2. An a priori evaluation... 

Classification of mechanisms might help to understand complicated processes, but quite often it might be misleading. There are many exceptions to each of the proposed mechanisms, and one can always criticize a proposed mechanism on the grounds of these exceptions. Therefore, playing the devil s advocate, we shall classify the process of protein biogenesis in membranes into three mechanisms vectorial translation, vectorial processing and protein incorporation. 

Vectorial translation [31,32]. Polypeptides are made on membrane-bound polysomes. Many of these proteins are synthesized with a 16-30 amino acid extension at the NH2-terminus. This signal sequence is hydrophobic in nature. Protein synthesis and translocation, into or across the membrane, are obligatorily linked. Therefore, the transmembrane movement is co-translational and it is coupled to the elongation of the polypeptide chain. Consequently, the completed polypeptide chain is never present in the compartment where it is synthesized. The polypeptides that do not yet cross the membrane are shorter than the mature protein. Addition of inhibitors of protein synthesis immediately arrest movement of the polypeptide across the membrane. 

Vectorial translation — biogenesis of secretory vesicles and acetylcholine receptor... 

Fig. 12.3. Current model for vectorial translation depicted for one of the subunits of acetylcholine receptor. The various events are described in the text. ER, endoplasmic reticulum SRP, signal recognition particle.

The biogenesis of both acetylcholine receptor and chromaffin granules share several common properties. The specific polypeptides are synthesized and transported into the membrane by a vectorial translation process. The specific proteins are sorted out by the Golgi apparatus and eventually fuse with the plasma membrane via the secretory pathway. Yet the acetylcholine receptor functions on the plasma membrane, and therefore it should stay on this membrane for a long time (2-7 days). On the other hand, the function of chromaffin granules is to store neurotransmitters. Therefore they stay most of their lifetime inside the cell and their fusion with the plasma membrane is temporary. Soon after the secretion process, the constituents of the chromaffin granule membrane must be removed from the plasma membrane by endocytosis. 

Fig. 12.5. Biogenesis and assembly of cytochrome 6-c, complex in the inner mitochondrial membrane. Cytochrome fc-Cj complex contains at least five different subunits COREI (corl), COREII (corll), nonheme iron protein (Fe-S), cytochrome c, (cyt Cj), and cytochrome b (cyt b). Cytochrome f> is a mitochondrial gene product and is probably assembled into the inner membrane (IM) via vectorial translation by mitochondrial ribosomes. The other subunits are synthesized on cytoplasmic ribosomes as larger precursors. The precursors, perhaps in association with a cytoplasmic factor , are attached to receptors on the mitochondrial outer membrane (OM). The complex laterally diffuses to the junctions of the outer and inner membranes, and with the help of a hypothetical translocator the precursors are imported across the membrane. Pre-Corl, pre-Corll, and the pre-nonheme iron protein cross the two membranes, whereas cytochrome c, becomes anchored to the outer face of the inner membrane, facing the intermembrane space (IMS). Cytochrome b is assembled inside the inner membrane, and the nonheme iron protein and Corl and Corll are assembled into the matrix side of the inner membrane. The N-terminal extensions are removed by a soluble matrix protease. The N-terminal extension of cytochrome c, is removed in two steps the first is catalyzed by the matrix protease and the second probably by a protease located on the outer face of the inner membrane.

For a variety of reasons (not the least of which is the biological precedent just discussed), we decided to focus our attention on the possibility that mitochondrial proteins encoded by nuclear genes are vectorially translated by a class of cytoplasmic ribosomes directly associated with the outer mitochondrial membrane. 

We discussed earlier that one mechanism for the transfer of particular translation products across the membrane barriers is the process of vectorial translation as carried out by polysomes attached to the rough endoplasmic reticulum of secretory cells. An in vitro assay for this process has been devised by Redman and Sabatini. In order to carry out this assay, it is necessary to release labeled, nascent polypeptide chains from membrane-bound ribosomes by reaction with puromycin and assess their distribution between the medium and the membrane compartment. We followed this procedure to assay for vectorial release of nascent chains from bound 80 S polysomes attached to the outer mitochondrial membrane. 

From these data, we calculated that about half of the puromycin-released nascent chains were discharged vectorially. This result is in fair agreement with the values for the extent of vectorial translation obtained by others with preparations of rough endoplasmic reticulum from rat liver. ... 

Fig. 22. Model for vectorial translation of nascent polypeptides into mitochondria.

As vectors a, b and c we choose the three basis vectors that also serve to define the unit cell (Section 2.2). Any translation vector t in the crystal can be expressed as the vectorial sum of three basis vectors, t = ua + vb + wc, where u, v and w are positive or negative integers. 

So far we have considered only one shell of equivalent electrons, but the mathematical techniques discussed can be translated fairly simply to the case of complex atomic configurations. With LS coupling the wave functions of ni/ n/ 2...nulNu configuration are normally constructed by the vectorial coupling of orbital and spin momenta of all the shells ... 

Having identified the symmetries of the electronic distortion operators, we now determine the symmetries of the nuclear degrees of freedom. These are defined as the direct product of the positional representation with the symmetry of the translations [12,16]. The n-simplex is situated in a (n — 1)-dimensional space and thus will exhibit (n - 1) translations. The corresponding irrep is denoted as Ft-. One easily realizes that this will correspond to the (n — 1,1) irrep from the center of the simplex one can move in n different directions, but the vectorial sum of all these directions amounts to zero, hence the translational space has one degree of freedom less than the number of sites. The direct product can be decomposed in a standard way as follows ... 

A simple approach to protein description consists of representing a protein by a sequence of properties of its constituent amino acids. Each amino acid is described by one ore more properties and therefore the total number of protein descriptors is given by the product of the number of amino acids in the protein and the number of selected amino acid properties. As this number of descriptors increases very fast with the size of proteins, this approach is usually applied to small- and medium-size peptides. Moreover, in QSAR studies that require uniform-length descriptors, it can be used only to describe a series of peptide analogues, vhich are peptide sequences with the same length. To enable QSAR studies of peptide sequences with different length, some method is required that is able to translate the peptide sequences into vectorial descriptors with the same number of variables. For example, ACC transforms were applied to compress information about principal properties of amino acids into peptide sequences with different length. 

Translational, rotational, and vibrational energies may be expressed in terms of vectorial quantities. Translational and rotational velocities may be represented by vectors, that is by lines with definite lengths and directions, which may be resolved into components along three spatial axes of coordinates. Similarly a vibration has an axis and an amplitude, and it is resolvable. Each component of each type of motion is called a degree of freedom. To a reasonable degree of approximation the energy may be expressed as a sum of terms in each of which a characteristic molecular constant multiplies the square of a suitable coordinate. Thus kinetic energy of translation is where x, y, and z are the components of velocity... 

Yet, the translation vector equivalently becomes by operatorial and vectorial series expansion in left and right hand sides, respectively ... 

The membrane-bound bacteriorhodopsin system also provides an example of the illustration of electrochemical aspects of enzymic machineries which were not discussed in the context of this symposium. Membrane-bound enzyme systems are of special interest because they are oriented yielding a vectorial property which was stressed by Peter Mitchell already in the early sixties. The structural mobility of intramembrane molecules is restricted due to the lipid protein interaction. There might be controlled translational motion or rotational motion or in cases such as the purple membrane no gross motion at all. 

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