Entropic switch regulates myristate exposure in the HIV-1 matrix protein
- Chun Tang*,
- Erin Loeliger*,
- Paz Luncsford*,
- Isaac Kinde*,
- Dorothy Beckett†, and
- Michael F. Summers*,‡
- *Howard Hughes Medical Institute and Department of Chemistry and Biochemistry, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250-5398; and †Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742-2021
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Edited by Adriaan Bax, National Institutes of Health, Bethesda, MD, and approved November 13, 2003 (received for review September 4, 2003)
Abstract
The myristoylated matrix protein (myr-MA) of HIV functions as a regulator of intracellular localization, targeting the Gag precursor polyprotein to lipid rafts in the plasma membrane during virus assembly and dissociating from the membrane during infectivity for nuclear targeting of the preintegration complex. Membrane release is triggered by proteolytic cleavage of Gag, and it has, until now, been believed that proteolysis induces a conformational change in myr-MA that sequesters the myristyl group. NMR studies reported here reveal that myr-MA adopts myr-exposed [myr(e)] and -sequestered [myr(s)] states, as anticipated. Unexpectedly, the tertiary structures of the protein in both states are very similar, with the sequestered myristyl group occupying a cavity that requires only minor conformational adjustments for insertion. In addition, myristate exposure is coupled with trimerization, with the myristyl group sequestered in the monomer and exposed in the trimer (K assoc = 2.5 ± 0.6 × 108 M–2). The equilibrium constant is shifted ≈20-fold toward the trimeric, myristate-exposed species in a Gag-like construct that includes the capsid domain, indicating that exposure is enhanced by Gag subdomains that promote self-association. Our findings indicate that the HIV-1 myristyl switch is regulated not by mechanically induced conformational changes, as observed for other myristyl switches, but instead by entropic modulation of a preexisting equilibrium.
Footnotes
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↵ ‡ To whom correspondence should be addressed. E-mail: summers{at}hhmi.umbc.edu.
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This paper was submitted directly (Track II) to the PNAS office.
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Abbreviations: CA, capsid protein; HSQC, heteronuclear single quantum coherence; MA, matrix protein; myr-MA, myristoylated HIV-1 MA; myr(–), unmyristoylated; myr(s), myristate sequestered state; myr(e), myristate exposed state; NC, nucleocapsid protein; NOE, nuclear Overhauser effect; NES, nuclear export signal; NLS, nuclear localization signal.
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Data deposition: The NMR chemical shift assignments for myr-MA have been deposited at the BioMagResBank, http://bmrb.wisc.edu (accession no. 5960). Coordinates for 20 conformers of the myristoylated matrix protein have been deposited in the Protein Data Bank, www.rcsb.org (PDB ID code 1UPH).
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See Commentary on page 417.
- Copyright © 2004, The National Academy of Sciences
