Protein matrix

Oberhauser et al., 1998] Oberhauser, A. F., Marszalek, P. E., Erickson, H., and Fernandez, J. The molecular elasticity of tenascin, an extracellular matrix protein. Nature. In Press. 

Cathepsins are intracellular proteinases that reside within lysosomes or specific intracellular granules. Cathepsins are used to degrade proteins or pqffides that are internalised from the extracellular space. Some cathepsins such as cathepsin-G or cathepsin-K may be released from the cell to degrade specific extracellular matrix proteins. All cathepsins except cathepsin-G (serine) and cathepsin-D (aspartyl) are cysteine proteinases. 

Reelin is an extracellular matrix protein, which is secreted by neuronal cells and binds to two lipoprotein receptors (VLDLR and ApoER2) that relay the Reelin signal inside target neurons by docking the tyrosine kinase adapter disabled-1 (Dabl). This allows neurons to complete migration and adopt their ultimate positions in laminar structures in the central nervous system. In... 

Fatal hereditary disorder that typically presents in the neonatal period. Clinical features include an array of hepatic, renal and neurological dysfunctions. Patients with Zellweger syndrome rarely survive the first year of life. The disease is caused by mutations in the Pex proteins leading to an defective import of peroxisomal matrix proteins and consequently to a loss of most peroxisomal metabolic pathways. 

It is of interest that proteins termed motility factors (55-70 kD) are secreted by fetal cells and some tumor cells. These proteins act as autocrine factors and stimulate rapid movement by these cells. Motility factors induce the formation of cell processes that are packed with actin filaments and have an increased number of receptors for the matrix proteins laminin and fibronectin. The latter enhance the ability of the cells to bind to the extracellular matrix. Thus, it is likely that motility factors influence the organization of the cytoskeleton through changes taking place at the cell surface (reviewed by Warn and Dowrick, 1989). 

The secretion of extracellular matrix proteins is also a function of smooth muscle cells but, since it occurs concurrently with other activities, it does not seem to constitute a physiological state. However, the fraction of the cellular resources which are devoted to it must be regulated these regulatory mechanisms are virtually unknown. In addition, it should be anticipated that autocrine activity occurs as well, involving peptides, prostaglandins, cytokines, and nitric oxide. 

Matrix proteins must pass from cytosolic polyribosomes through the outer and inner mitochondrial membranes to reach their destination. Passage through the two membranes is called translocation. They have an amino terminal leader sequence (presequence),... 

Translocation is believed to occur posttranslation-ally, after the matrix proteins are released from the cytosolic polyribosomes. Interactions with a number of cytosolic proteins that act as chaperones (see below) and as targeting factors occur prior to translocation. 

Osteoclasts are multinucleated cells derived from pluripotent hematopoietic stem cells. Osteoclasts possess an apical membrane domain, exhibiting a ruffled border that plays a key role in bone resorption (Figure 48-12). A proton-translocating ATPase expels protons across the ruffled border into the resorption area, which is the microenvironment of low pH shown in the figure. This lowers the local pH to 4.0 or less, thus increasing the solubility of hydroxyapatite and allowing demineralization to occur. Lysosomal acid proteases are released that digest the now accessible matrix proteins. 

Increased covalent binding to vascular matrix proteins... 

Matrix metalloproteinases Zinc-dependent enzymes capable of degrading extracellular matrix proteins, including connective tissue. 

Volck, B., Price, P.A., Johansen, J.S., Sorensen, O., Benfield, T.L., Nielsen, H.J., Calafat, J. and Borregaard, N. (1998) YKL-40, a mammalian member of the chitinase family, is a matrix protein of specific granules in human neutrophils. Proceedings of the Association of American Physicians 110, 351-360. 

Bennett, J., in Cell adhesion molecules and matrix proteins (S. Mousa Ed.), p. 29. Springer-Verlag, Berlin (1998). 

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