Non-proteolytic Functions

The function and mechanism of Ubcl3 are discussed in more detail below (Section 5.7). 

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UBC13 153 Rad5 [73] Heterodimerizes with Mms2 (UEV) [72] DNA-damage tolerance [72] via polyubiquiti-nation of PCNA [65] (non-proteolytic function)... 

TNF- a was first purified from conditioned medium from HL-60 cells. It has a relative molecular mass of 17 kDa when analysed by SDS-PAGE, but 45 kDa when analysed by gel filtration. Thus, the molecule exists as a non-glycosylated trimer with a pi of 5.3. Each monomer comprises 157 amino acids and contains two cysteine residues that form a disulphide bridge. Trimer formation appears to be due to noncovalent interactions between the monomers. Human TNF-a is synthesised as a 233-amino-acid protein that is proteolytically cleaved during processing. Whilst the 17-kDa form is readily secreted (and hence can function as an extracellular mediator), a 26-kDa transmembrane form has also been identified. This form of TNF-a may thus function in cytotoxicity resulting from cell-cell contact. 

The direct acylation and methylation of the free 01-NH2 groups of proteins have been proposed to be useful in providing resistance toward proteolytic attacks. Although the basis for this explanation is not always readily apparent from the known specificities of proteases, it may be valid in some cases. Thus, the acetylated N-terminus of a-crystallin, the major protein found in eye lens (17), is presumably important for the protein to survive in an environment rich in leucine aminopeptidase. On the other hand, it is difficult to rationalize that the acetylated N-terminus of bovine pancreatic a-amylase is in any way responsible for the fact that the enzyme is exceedingly stable against tryptic and chymotryptic digestion (18). The function of the acetylation is, in this case, as obscure as is the basis on which a-amylase is selected for acetylation among the many non-acetylated companion pancreatic proteins. 

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