Schematic representation of targeted amino acid positions in the _E subunit for site-directed mutagenesis. Residues are numbered as done previously.6 Arrows or simple vertical lines indicate positions of those cysteines converted, either singly or in combination, to serine or alanine by site-directed mutagenesis. (A) The three regions of the _E chain: the N-terminus (NT) containing a large -helical segment, the so-called “C” region, and the C-terminal globular VI-domain (_EC domain); (B) the VI-domain, potential glycosylation sites in the wild-type sequence at Asn667 and Asn812 are marked, respectively, with closed and open diamonds. The glycosylation site at Asn791 introduced by the Cys793 → Ser change is marked with a striped diamond. Putative loops connecting cysteines _E613/644 as well as _E780/793 are drawn by analogy with the intrachain loops formed by homologous cysteines in the βC and γC domains.