Reconsidering the early steps of protein secretion.

We have reviewed a unique mutation affecting the hydrophilic segment of the lambda-receptor signal sequence. Considering the implications of this mutation, we have proposed a model depicting early steps of protein secretion. The salient, novel features of this model are as follows. 1) The model explicitly states that the LamB protein must initiate the export process to be synthesized. Commitment to synthesis occur subsequent to an interaction between the hydrophilic segment of the signal sequence and a membrane export site, the initial interaction between the nascent secreted protein and the membrane. Implicit in this concept is obligatory co-translational secretion of lambda receptor. 2) We suggest the existence of a "stop translation" sequence. The role of this sequence is to halt translation in order to allow sufficient time for the hydrophilic portion of the signal sequence to initiate export by interacting with a membrane receptor. 3) We suggest that at least part of the "stop translation" sequence is located down-stream from the signal sequence, after residue 15 of the mature LamB protein. 4) We further speculate, albeit in the absence of direct evidence, that the hydrophobic portion of the signal sequence may also be part of the "stop translation" sequence.