Peroxisomal matrix protein

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. 

K N., Wenzel, T. J., Rangell, L, Keller, G. A., and Subramani, S. Pex22p of Pichia pastoris, essential for peroxisomal matrix protein import, anchors the ubiquitin-conjugating enzyme, Pex4p, on the peroxisomal membrane. J. Cell Biol. 1999, 346, 99-112. 

Zellweger syndrome Is a llpid storage disorder caused by impaired peroxisome biogenesis due to deficiency or functional defect of one of eleven proteins involved in the complex mechanism of peroxisomal matrix protein import and assembly of the organelle. 

Both the integral membrane proteins of peroxisomes as well as the peroxisomal matrix proteins are synthesised on free polyribosomes and are specifically targeted to peroxisomes via dedicated peroxisomal targeting signals. The biogenesis of peroxisomes follows a sequential pattern, which involves first the insertion of peroxisomal membrane proteins into the membrane of the pre-peroxisomal structure, derived from the endoplasmic reticulum, followed by insertion of the various matrix proteins. 

The import of catalase and other proteins into rat liver peroxisomes can be assayed in a cell-free system similar to that used for monitoring mitochondrial protein Import (see Figure 16-25). By testing various mutant catalase proteins in this system, researchers discovered that the sequence Ser-Lys-Leu (SKL in one-letter code) or a related sequence at the C-terminus was necessary for peroxisomal targeting. Further, addition of the SKL sequence to the C-termlnus of a normally cytosolic protein leads to uptake of the altered protein by peroxisomes in cultured cells. All but a few of the many different peroxisomal matrix proteins bear a sequence of this type, known as peroxisomal-targeting sequence 1, or simply PTSl. 

A few peroxisomal matrix proteins such as thiolase are synthesized as precursors with an N-terminal uptaketargeting sequence known as PTS2. These proteins bind to a different cytosolic receptor protein, but otherwise import is thought to occur by the same mechanism as for PTS1-containing proteins. 

Most peroxisomal matrix proteins contain a C-termlnal PTSl targeting sequence a few have an N-termlnal PTS2 targeting sequence. Neither targeting sequence is cleaved after Import. 

Translocation of matrix proteins across the peroxisomal membrane depends on ATP hydrolysis. Many peroxisomal matrix proteins fold in the cytosol and traverse the membrane in a folded conformation. 

Proteins destined for the peroxisomal membrane contain different targeting sequences than peroxisomal matrix proteins and are imported by a different pathway. 

Note that the SLO treatment permeabilizes only cell membranes the membranes of peroxisomes remain intact as judged by the impermeability to antibodies (Wend-land and Subramani, 1993a). To allow access of antibodies to peroxisomal matrix proteins the peroxisomal membrane is permeabilized by treatment with Triton X-100. 

Owing to the small number of known peroxisomal membrane proteins, the signals (mPTSs) that direct them to the membrane are not well characterized. In one example, mPTS is found in the fourth loop (20 residues) of a 6 membrane-spanning protein. The loop faces to the matrix side. In another example, mPTS was found in the N-terminal 40-residue segment on the matrix side. Both of them contain a 5-residue stretch rich in basic amino acids and seem to be a part of the mPTS. 

Peroxisome (matrix) C-terminus (most proteins) N-terminus (few proteins) No PTSl signal (Ser-Lys-Leu) at extreme C-terminus PTS2 signal at N-terminus... 

Cytosolic Receptor Targets Proteins with an SKL Sequence at the C-Terminus into the Peroxisomal Matrix... 

Peroxisomal Membrane and Matrix Proteins Are Incorporated by Different Pathways... 

Autosomal recessive mutations that cause defective peroxisome assembly occur naturally in the human iU population. Such defects can lead to severe impairment of many organs and to death. In Zellweger syndrome and related disorders, for example, the transport of many or all proteins into the peroxisomal matrix is impaired newly synthesized peroxisomal enzymes remain in the cytosol and... 

All proteins destined for the peroxisomal matrix bind to a c3d osollc receptor, which differs for PTSl- and PTS2-... 

Suppose that you have identified a new mutant cell line that lacks functional peroxisomes. Describe how you could determine experimentally whether the mutant Is primarily defective for insertion/assembly of peroxisomal membrane proteins or matrix proteins. 

Subramani, S. (1992) Targeting of proteins into the peroxisomal matrix. J. Membrane Biol. 125, 99-106. 

Peroxisomal proteins have been found to be synthesised on jree polyribosomes. This applies to both peroxisomal matrix as well as membrane proteins. These and other findings led Lazarow and Fujiki (2) to formulate a model for peroxisome biogenesis which is generally accepted. The principal features of this model are ... 

Peroxisomal matrix and membrane proteins are synthesised on free polyribosomes 2. The newly synthesised proteins are posttranslationally imported from the cytosol into pre-existing peroxisomes 3. Import of new polypeptides expands the peroxisomal compartment making them grow until they reach a critical size which results either in division of peroxisomes into two daughter peroxisomes or in budding from the peroxisomal reticulum followed by subsequent growth. 

N- and C-terminal domains. " The crystal structure of pig heart L-3-hydroxyacyl-CoA dehydrogenase was published a decade ago, but the catalytic mechanism of this type of dehydrogenase was not known until we identified a conserved histidine as the catalytic residue. L-3-Hydroxyacyl-CoA dehydrogenases are associated with the C-terminal region of multifunctional proteins except for those in the mitochondrial matrix (see Table 1). In contrast, D-3-hydroxyacyl-CoA dehydrogenases are associated with the N-terminal region of peroxisomal multifunctional proteins. ... 

Transport signals can be of the import or the export type. Import signals are contained in proteins that are transported into the individual compartments of mitochondria (matrix, inner membrane, intramembrane compartment, outer membrane), peroxisomes (lumen, boundary membrane) and into the interior of... 

Fatty acid utilized by muscle may arise from storage triglycerides from either adipose tissue depot or from lipid stores within the muscle itself. Lipolysis of adipose triglyceride in response to hormonal stimulation liberates free fatty acids (see Section 9.6.2) which are transported through the bloodstream to the muscle bound to albumin. Because the enzymes of fatty acid oxidation are located within subcellular organelles (peroxisomes and mitochondria), there is also need for transport of the fatty acid within the muscle cell this is achieved by fatty acid binding proteins (FABPs). Finally, the fatty acid molecules must be translocated across the mitochondrial membranes into the matrix where their catabolism occurs. To achieve this transfer, the fatty acids must first be activated by formation of a coenzyme A derivative, fatty acyl CoA, in a reaction catalysed by acyl CoA synthetase. 

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