A multimeric membrane protein reveals 14-3-3 isoform specificity in forward transport in yeast. Traffic 7:903–916

Arginine (Arg)-based endoplasmic reticulum (ER) locali-zation signals are sorting motifs involved in the quality control of multimeric membrane proteins. They are dis-tinct from other ER localization signals like the C-terminal di-lysine [-K(X)KXX] signal. The Pmp2p isoproteolipid, a type I yeast membrane protein, reports faithfully on the activity of sorting signals when fused to a tail containing either an Arg-based motif or a-KKXX signal. This reporter reveals that the Arg-based ER localization signals from mammalian Kir6.2 and GB1 proteins are functional in yeast. Thus, the machinery involved in recognition of Arg-based signals is evolutionarily con-served. Multimeric presentation of the Arg-based signal from Kir6.2 on Pmp2p results in forward transport, which requires 14-3-3 proteins encoded in yeast by BMH1 and BMH2 in two isoforms. Comparison of a strain without any 14-3-3 proteins (Dbmh1Dbmh2) and the individual Dbmh1 or Dbmh2 shows that the role of 14-3-3 in the trafficking of this multimeric Pmp2p reporter is isoform-specific. Efficient forward transport requires the pre-sence of Bmh1p. The specific role of Bmh1p is not due to differences in abundance or affinity between the iso-forms. Our results imply that 14-3-3 proteins mediate forward transport by a mechanism distinct from simple masking of the Arg-based signal. Key words: 14-3-3 proteins, Arg-based ER localization signal, forward transport, multimeric membrane protein, yeast Received 7 February 2006, revised and accepted fo