Figure 3.

Accessibility of introduced cysteines to 5-IAF. Experiments like those in Fig. 2 ▶ were performed to determine relative accessibility to 5-IAF of cysteine sulfhydryls at each of the indicated positions in membrane-embedded Trg. Mean values of accessibility (three independent membrane preparations) with standard deviations are plotted by residue number from cytoplasm to periplasm for the TM1 (A) and TM2 regions (B). Positions of charged residues in the wild-type protein are marked by “+” or “−” along the abscissa, and the sequence of 19 consecutive residues with minimal accessibility is marked by divergent arrows and labeled “19 aa.” Position 48 is marked in panel A by an asterisk (*) to indicate that the low accessibility is likely the result of bracketing negative charges restricting access by the negatively charged reagent (see text). Because of low receptor content, it was not possible to make reliable measurements for positions 208 and 229.