Role of NH₂-terminal positively charged residues in establishing membrane protein topology

The paramyxovirus HN polypeptide is a model type II membrane protein, containing an internal uncleaved signal/anchor (S/A) and is oriented in the membrane with an NH₂-terminal cytoplasmic domain and COOH-terminal ectodomain (N(cyt) topology). To test the role of NH₂-terminal positively charged residues in directing the HN membrane topology, the 3 arginine (Arg) residues within the 17-amino-acid NH₂-terminal domain were systematically converted to a glutamine or glutamate, and the topology of the mutant proteins was examined after expression in CV-1 cells. The data indicate that: (i) each of the NH₂-terminal Arg residues contributes to the signal directing proper HN topology, since substitutions in any of the three positions resulted in ~13-23% inversion into the N(exo) form; (ii) substitutions in the Arg directly flanking the signal/anchor domain resulted in slightly more inversion than those which were located more distally; and (iii) substitution with a negatively charged glutamate led to more inversion than did replacement with an uncharged glutamine. The effect of a single Arg to Glu substitution on the HN topology was enhanced when present in the context of a truncated NH₂-terminal cytoplasmic tail (3 residues). A comparison of the sequences flanking the signal/anchor of well documented type III proteins showed that the majority of these proteins contain a negatively charged residue flanking the NH₂-terminal side. An exception to this rule is the NB protein which contains a single positively charged Arg residue in this position. A chimeric protein containing the NB ectodomain and the HN S/A and HN ectodomain lead to a significant fraction (70%) of the chimeric protein adopting type II topology suggesting that the positive charge flanking the S/A domain is important for establishing type II topology. These data are discussed in the context of the loop model for the biogenesis of integral membrane proteins and the possible signals necessary for establishing differing orientations.