ARABIDOPSIS MBP1 GENE ENCODES A CONSERVED UBIQUITIN RECOGNITION COMPONENT OF THE 26S PROTEASOME

Multiubiquitin chain attachment is a key step leading to the selective degradation of abnormal polypeptides and many important regulatory pr oteins by the eukaryotic 26S proteasome. However, the mechanism by whi ch the 26S complex recognizes this posttranslational modification is u nknown, Using synthetic multiubiquitin chains to probe an expression l ibrary for interacting proteins, we have isolated an Arabidopsis cDNA, designated MBP1, that encodes a 41-kDa acidic protein exhibiting high affinity for chains, especially those containing four or more ubiquit ins. Based on similar physical and immunological properties, multiubiq uitin binding affinities, and peptide sequence, MBP1 is homologous to subunit 5a of the human 26S proteasome. Structurally related proteins also exist in yeast, Caenorhabditis, and other plant species, Given th eir binding properties, association with the 26S proteasome, and wides pread distribution, MBP1, S5a, and related proteins likely function as essential ubiquitin recognition components of the 26S proteasome.