PRODUCT OF A NEW GENE, SYD, FUNCTIONALLY INTERACTS WITH SECY WHEN OVERPRODUCED IN ESCHERICHIA-COLI

A mutant form of SecY, SecY(-d)1, was previously suggested to sequeste r a component(s) of the protein translocator complex. Its synthesis fr om a plasmid leads to interference with protein export in Escherichia coil. SecE is a target of this sequestration, and its overproduction c ancels the export interference. We now report that overexpression of a nother gene, termed syd, also suppresses secY(-d)1. The nucleotide seq uence of syd predicted that it encodes a protein of 181 amino acid res idues, which has been identified by overproduction, purification, and determination of the amino terminal sequence. Cell fractionation exper iments suggested that Syd is loosely associated with the cytoplasmic s urface of the cytoplasmic membrane. SecY may be involved in the membra ne association of Syd since the association is saturable, the extent o f which depends on the overproduction of SecY. SecY is rapidly degrade d in vivo unless its primary partner, SecE, is sufficiently available. Overproduction of Syd was found to stabilize oversynthesized SecY. Ho wever, Syd cannot stabilize the SecY(-d)1 form of SecY. Thus, in the p resence of both secY(+) and secY(-d)1, Syd increases the effective Sec Y(+)/SecY(-d)1 ratio in the cell and cancels the dominant interference by the latter. We also found that overproduction of Syd dramatically inhibits protein export in the secY24 mutant cell in which SecY-SecE i nteraction has been weakened. These results indicate that Syd, especia lly when it is overproduced, has abilities to interact with SecY. Poss ible significance of such interactions is discussed in conjunction wit h the apparent lack of phenotypic consequences of genetic disruption o f syd.