PHOTOLABILE DERIVATIVES OF I-125 APAMIN - DEFINING THE STRUCTURAL CRITERIA REQUIRED FOR LABELING HIGH AND LOW-MOLECULAR-MASS POLYPEPTIDES ASSOCIATED WITH SMALL-CONDUCTANCE CA2-ACTIVATED K+ CHANNELS()

The structure of apamin-sensitive Ca2+-activated K+ channels has been investigated using high-affinity, photolabile azidoaryl derivatives of I-125-[alpha-formyl-Cys(1)] apamin and I-125-[epsilon-formyl-Lys(4)]- apamin. Labeling, patterns suggest that similar structural constraints are required for labeling analogous polypeptides associated with dist inct channel subtypes. When photoprobes are coupled at the epsilon-ami no-Lys(4) position of apamin, comparable low molecular mass (similar t o 30 kDa) polypeptides are efficiently labeled on either brain or live r plasma membranes, irrespective of the structure of the photoprobe. H owever, when photoprobes are coupled at the alpha-amino-Cys(1) positio n of apamin, the pattern of labeling on both brain and liver plasma me mbranes varies, depending upon the length of the spacer arm incorporat ed into the photoprobe. Spacer arms of approximately 8-9 Angstrom effi ciently label only high molecular mass polypeptides (86, 59 kDa), acco mpanied by weak, variable labeling of a 44-kDa component. A shorter sp acer arm (5.7 Angstrom) results in feeble labeling of 86- and 59-kDa p olypeptides and barely detectable labeling of 44- and similar to 30-kD a polypeptides. In contrast, a long spacer arm (12.8 Angstrom) efficie ntly labels only similar to 30-kDa polypeptides. These findings point to close similarities in the topography of the I-125-apamin binding si te present on pharmacologically distinct subtypes of apamin-sensitive Ca2+-activated K+ channels and indicates that heterooligomeric associa tion of high and low molecular mass polypeptide subunits may be a gene ral structural feature of members belonging to this family of K+ chann els.