Degradation signals within both terminal domains of the cauliflower mosaic virus capsid protein precursor

Abstract

SummaryTargeted protein degradation plays an important regulatory role in the cell, but only a few protein degradation signals have been characterized in plants. Here we describe three instability determinants in the termini of the cauli¯ower mosaic virus (CaMV) capsid protein precursor, of which one is still present in the mature capsid protein p44. A modi®ed ubiquitin protein reference technique was used to show that these motifs are still active when fused to a heterologous reporter gene. The N-terminus of p44 contains a degradation motif characterized by proline, glutamate, aspartate, serine and threonine residues (PEST), which can be inactivated by mutation of three glutamic acid residues to alanines. The signals from the precursor do not correspond to known degradation motifs, although they confer high instability on proteins expressed in plant protoplasts. All three instability determinants were also active in mammalian cells. The PEST signal had a signi®cantly higher degradation activity in HeLa cells, whereas the precursor signals were less active. Inhibition studies suggest that only the signal within the N-terminus of the precursor is targeting the proteasome in plants. This implies that the other two signals may target a novel degradation pathway.

Selective autophagy limits cauliflower mosaic virus infection by NBR1-mediated targeting of viral capsid protein and particles

“…3F). Of note, P4 has previously been shown to contain instability determinants in the N-and C-terminal domains (25), which potentially could be responsible for NBR1-mediated targeting and subsequent degradation. Indeed, both N-and C-terminal regions, but not the middle (M) part of P4, interacted with NBR1 (Fig.…”

Ubiquitin and Plant Viruses, Let’s Play Together!

“…In contrast, the precursor of the caulimovirus Cauliflower mosaic virus (CaMV) coat protein (CP) was reported to contain three specific instability determinants (Karsies et al, 2001), which make this protein likely to be the target of a regulatory process. One uncharacterized degradation signal targets the CP precursor for degradation by the proteasome, whereas the other two, which display characteristics of PESTrelated sequences (Rechsteiner and Rogers, 1996), seem to induce a proteasome-independent degradation pathway.…”

Mutation of Capsid Protein Phosphorylation Sites Abolishes Cauliflower Mosaic Virus Infectivity

“…The instability in protoplasts of CP56 mutants containing the phosphoserines (12) may hint that a host protease is involved in CP44 processing. However, this processing may also result from degradation of the host machinery (12). CaMV CP56 processing certainly merits further attention.…”

“…CP44, but not CP39 or CP37, contains an acidic N-terminal sequence, which also includes serine, threonine, and proline residues, a combination that frequently renders proteins unstable (12). The acidic domain is followed by a short nuclear localization signal (13), an assembly domain (1), and a large cluster of basic amino acids involved in nucleic acid binding (1,4), including a Zn finger motif specifying interaction with a packaging signal located on the CaMV RNA leader sequence (7).…”

“…CP44 starts with amino acid 77 of the precursor protein (16); however, neither the N termini of CP39 and CP37 nor the C termini of any of these proteins are precisely known. Additional domains characterized by acidic residues, serine, and threonine and leading to protein instability make up the N and C termini of CP56 (12).…”