At MBP ‐1, an alternative translation product of LOS 2 , affects abscisic acid responses and is modulated by the E 3 ubiquitin ligase A t SAP 5

Summary The LOS 2 gene in Arabidopsis encodes an enolase with 72% amino acid sequence identity with human ENO 1. In mammalian cells, the α‐enolase ( ENO 1 ) gene encodes both a 48 k D a glycolytic enzyme and a 37 k D a transcriptional suppressor protein that are targeted to different cellular compartments. The tumor suppressor c‐ myc binding protein ( MBP ‐1), which is alternatively translated from the second start codon of ENO 1 transcripts, is preferentially localized in nuclei while α‐enolase is found in the cytoplasm. We report here that an Arabidopsis MBP ‐1‐like protein (At MBP ‐1) is alternatively translated from full‐length LOS 2 transcripts using a second start codon. Like mammalian MBP ‐1, this truncated form of LOS 2 has little, if any, enolase activity, indicating that an intact N‐terminal region of LOS 2 is critical for catalysis. At MBP ‐1 has a short half‐life in vivo and is stabilized by the proteasome inhibitor MG 132, indicating that it is degraded via the ubiquitin‐dependent proteasome pathway. Arabidopsis plants that over‐express At MBP ‐1 are hypersensitive to abscisic acid ( ABA ) during seed germination and show defects in vegetative growth and lateral stem development. At MBP ‐1 interacts directly with the E 3 ubiquitin ligase At SAP 5 and co‐expression of these proteins resulted in destabilization of At MBP ‐1 in vivo and abolished the developmental defects associated with At MBP ‐1 over‐expression. Thus, At MBP ‐1 is as a bona fide alternative translation product of LOS 2 . Accumulation of this factor is limited by ubiquitin‐dependent destabilization, apparently mediated by At SAP 5.