Structural Domains in Phospholemman

A Possible Role for the Carboxyl Terminus in Channel Inactivation

From the Department of Medicine and the Krannert Institute of Cardiology (Z.C., L.R.J.), Indiana University School of Medicine, Indianapolis, Ind; the Dupont Merck Pharmaceutical Co Applied Biotechnology (J.J.O.), Wilmington, Del; the Departments of Medicine and Molecular Physiology and Biological Physics (J.R.M.), University of Virginia Health Sciences Center, Charlottesville, Va; and the Program in Molecular and Cellular Cardiology (S.E.C.), Department of Medicine, Wayne State University School of Medicine, Detroit, Mich.

Correspondence and reprint requests to Steven E. Cala, PhD, Cardiology Research Division, 421 East Canfield, Room 1107, Detroit, MI 48201. E-mail jones{at}kimail.dmed.iupui.edu

Abstract—Phospholemman (PLM) is a small (72–amino acid) transmembrane protein found in cardiac sarcolemma that is a major substrate for several protein kinases in vivo. Detailed structural data for PLM is lacking, but several studies have described an ion conductance that results from PLM expression in oocytes. Moreover, addition of purified PLM to lipid bilayers generates similar ion currents, suggesting that the PLM molecule itself might be sufficient for channel formation. To provide a framework for understanding the function of PLM, we investigated PLM topology and structure in sarcolemmal membrane vesicles and analyzed purified recombinant PLM. Immunoblot analyses with site-specific antibodies revealed that the extracellular segment (residues 1 to 17) exists in a protected configuration highly resistant to proteases, even in detergent solutions. The intracellular portion of the molecule (residues 38 to 72), in contrast, was highly susceptible to proteases. Trypsin treatment produced a limit peptide (residues 1 to 43), which showed little change in electrophoretic mobility in SDS gels and retained the ion-channel activity in lipid bilayers that is characteristic of the full-length protein. In addition, we found that conductance through PLM channels exhibited rapid inactivation during depolarizing ramps at voltages greater than ±50 mV, Channels formed by trypsinized PLM or recombinant PLM 1–43 exhibited dramatic reductions in voltage-dependent inactivations. Our data point to distinct domains within the PLM molecule that may correlate with functional properties of channel activity observed in oocytes and lipid bilayers.

Key Words: phospholemman • sarcolemma • ion channel • topology

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