Abstract
Ssh1p of Saccharomyces cerevisiae is related in sequence to Sec61p, a general receptor for signal sequences and the major subunit of the channel that guides proteins across the membrane of the endoplasmic reticulum. The split-ubiquitin technique was used to determine whether Ssh1p serves as an additional receptor for signal sequences in vivo. We measured the interactions between the N(ub)-labeled Ssh1p and C(ub)-translocation substrates bearing four different signal sequences. The so-determined interaction profile of Ssh1p was compared with the signal sequence interaction profile of the correspondingly modified N(ub)-Sec61p. The assay reveals interactions of Ssh1p with the signal sequences of Kar2p and invertase, whereas Sec61p additionally interacts with the signal sequences of Mfalpha1 and carboxypeptidase Y. The measured physical proximity between Ssh1p and the beta-subunit of the signal sequence recognition particle receptor confirms our hypothesis that Ssh1p is directly involved in the cotranslational translocation of proteins across the membrane of the endoplasmic reticulum.