A transmembrane form of annexin XII detected by site-directed spin labeling

A transmembrane form of annexin XII detected by site-directed spin labeling

^*Jules Stein Eye Institute and Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095; and ^‡Department of Physiology and Biophysics, University of California, Irvine, CA 92697

Communicated by Irwin A. Rose, University of California, Irvine, CA (received for review September 1, 1998)

Abstract

Previous studies of the annexin family of Ca²⁺ binding proteins identified a soluble monomer in the absence of Ca²⁺ and a trimer adsorbed on the membrane surface in the presence of Ca²⁺. On the basis of site-directed spin-labeling studies of annexin XII at low pH, we now report a membrane-inserted form of the protein with a dramatically different structure. The data suggest that upon insertion a continuous transmembrane α-helix is reversibly formed from a helix–loop–helix motif in the solution structure. Other regions with similar membrane-insertion potential were identified in the amino acid sequence, and we propose that the corresponding helices come together to form an aqueous pore that mediates the ion channel activity reported for several annexins.

Footnotes

↵ † R.L. and J.M.I. contributed equally to this work.
↵ § To whom reprint requests should be addressed. e-mail: hhaigler{at}uci.edu and hubbellw{at}jsei.ucla.edu.
ABBREVIATION:

NiEDDA,

nickle(ethylenediaminediacetic acid)