Secretion of a type II integral membrane protein induced by mutation of the transmembrane segment

Signal peptide/membrane anchor (SA) domains of type II membrane proteins initiate the translocation of downstream polypeptides across the endoplasmic reticulum (ER) membrane. In contrast with signal peptides, however, SA domains are not cleaved by signal peptidase and thus anchor the protein in the membrane. In the present study we have introduced mutations in the SA domain of neprilysin (neutral endopeptidase-24.11; NEP) to identify structural elements that would favour the processing of SA domains by signal peptidase. Mutants of full-length and truncated (without cytoplasmic domain) protein were constructed by substitution of the sequences SQNS, QQTT or YPGY for VTMI starting at position 15 of the NEP SA domain. In addition, a Pro residue was substituted for Thr at position 16 of the SA domain. The rationale for the use of these sequences was decided from our previous observation that substitution in the NEP SA domain of the sequence SQNS, which is polar and has alpha-helix-breaking potential, could promote SA domain processing under certain conditions (Roy, Chatellard, Lemay, Crine and Boileau (1993) J. Biol. Chem. 268. 2699-2704; Yang. Chatellard, Lazure, Crine and Boileau (1994) Arch. Biochem. Biophys. 315, 382-386). The QQTT sequence is polar but, according to secondary structure predictions, is compatible with the alpha-helix structure of the NEP SA domain. The YPGY sequence and single Pro residue are less polar and have alpha-helix-breaking potential. The predicted effects of these mutations on the structure of the NEP SA domain were confirmed by CD analysis of 42-residue peptides encompassing the hydrophobic segment and flanking regions. Wild-type and mutated proteins were expressed in COS-I cells and their fate (membrane-bound or secreted) was determined by immunoblotting and by endoglycosidase digestions. Our biochemical and structural data indicate that: (I) the cytosolic domain of NEP restricts the conformation of the SA domain because mutants not secreted in their full-length form are secreted in their truncated form; (2) alpha-helix-breaking residues are not a prerequisite for cleavage; (3) the presence, in close proximity to a putative signal peptidase cleavage site, of a polar sequence that maintains the alpha-helical structure of the SA domain is sufficient to promote cleavage. Furthermore pulse chase studies suggest that cleavage is performed in the ER by signal peptidase and indicate that cleavage is not a limiting step in the biosynthesis of the soluble form of the protein.