Destruction Box

Two of the four different E3 enzymes mentioned in the text are shown. The cyclosome/anaphase promoting complex (APC) ligates ubiquitin to regulatory and structural cell cycle proteins containing a destruction box as recognition signal (see also 13.3.2). The activity of the APC is thought to be controlled by phosphorylation. Eor simplicity, the subunits of APC are not shown. 

The cyclosome (or APC) is a high molecular weight complex that degrades proteins containing a specific recognition sequence, the destruction box (see chapter 13.2.4). Substrates are cell cycle regulators as e.g. cyclins, kinase inhibitors and spindle-associated proteins. Importantly, some forms of the cyclosome require phosphorylation in order to be active (fig. 2.15B). It is still unclear which of the many subunits carries the E3 enzyme activity. 

The APC possesses a ubiquitin-ligase activity with specificity for substrate proteins that contain a particular sequence, the destruction box (see 13.2.4). During and after mitosis, APC-mediated specific proteolysis is observed of the mitotic cyclins B and A and of a number of other regulatory proteins. All these proteins contain one or two copies of the destruction box. 

Proteins targeted for proteolysis have destruction boxes, i.e. specific recognition sequences for the proteolysis machinery, similar to the death domains of proteins destroyed in apoptosis (Chapter 13). 

Most substrates for APC-mediated ubiquitinylation carry a particular sequence, the destruction box. For B-type cyclins, the consensus sequence of the destruction box is R-ALGVN/D/EI-N. Deletion of the destruction box in cyclin B causes its stabilization. 

Peptide motifs. Proteins with certain homologous sequences are rapidly degraded. For example, proteins that have extended sequences containing proline, glutamate, serine, and threonine have half-lives of less than 2 hours. (PEST sequences are named for the one-letter abbreviations for these amino acids. See Table 5.1.) The cyclin destruction box is a set of homologous nine-residue sequences near the N-terminus of cyclins that ensures rapid ubiquination. 

The multisubunit anaphase-promoting complex (APC) is a ublqultln llgase that recognizes a conserved destruction box sequence in mitotic cycllns and promotes their polyublqultlnatlon, marking the proteins for rapid degradation by proteasomes. The resulting decrease in MPF activity leads to completion of mitosis. 

It has been shown that cyclin B has a short amino acid sequence near its /V-terminus that targets the protein for destruction between metaphase and anaphase (destruction of cyclin B is mediated by ubiqui-tin-mediated proteolysis). Removal of this sequence (known as a destruction box) renders the cyclin indestructible, and shows that cyclin destruction is necessary for chromosome decondensation, nuclear envelope reformation and cytokinesis, but not for sister chromatid segregation. 

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