HIV Envelope

The Envelope glycoprotein (Env) is the sole viral protein present on the surface of Human Immunodeficiency Virus-1 (HIV-1) virions. Env is synthesized as a 160 kDa precursor protein (gp160). It folds and trimerizes in the endoplasmic reticulum (ER) of the host cell, where it obtains ten disulfides and ~30 N-linked glycans depending on the viral isolate. In the Golgi complex, gp160 is cleaved by a cellular protease into a soluble subunit, gp120, and a transmembrane subunit, gp41. They remain non-covalently associated on the surface of infected cells and on virions. Together, the two Env subunits mediate viral entry: gp120 is responsible for binding to the receptor (CD4) and the coreceptor (CCR5 or CXCR4) on the host cell, and gp41 is needed for subsequent fusion of the viral and cellular membranes.

Folding of Env is slow to extremely slow dependent on the viral isolate: IIIB leaves the ER with a t1/2 of 30 min, while for LAI exit from the ER takes hours. Folding intermediates with few or non-native disulfides persist for hours after translation. Evidence for the employment of ER chaperones during the folding process of Env is its association with BiP, calnexin and calreticulin. Special to the folding process of Env is the late removal of its leader peptide with a t1/2 of ~15 min. for IIIB and ~30 min. for LAI. This occurs in a conformation-dependent fashion, since some initial folding of Env is required for leader peptide cleavage. Rate-limiting for folding of the gp160 precursor is the folding of gp120, probably because it harbors the majority of glycans and disulfide bonds. Therefore, folding of the gp120 soluble subunit alone largely reflects the folding of Env as a whole. Although slow, the folding process is productive: few if any Env molecules are degraded or end up in aggregates. Instead, the majority of Env reaches the native state.

Land A, Zonneveld D & Braakman I Folding of HIV-1 envelope glycoprotein involves extensive isomerization of disulfide bonds and conformation-dependent leader peptide cleavage. FASEB J. 2003; 17:1058-67. pdf
Land A & Braakman I Folding of the human immunodeficiency virus type 1 envelope glycoprotein in the endoplasmic reticulum. Biochimie 2001;83:783-90. pdf

We are currently investigating a number of conditions that could influence folding kinetics and efficiency, for instance, intracellular calcium levels, temperature changes etc. We also investigate the role of individual ER chaperones in the folding process of Envelope. To systematically assess the role of all individual disulfide bonds in Env folding and function, we currently analyze a complete series of mutants, where all cysteines were replaced by alanines, both individually and pair-wise.

• The HIV Env folding crew
  Ineke Braakman
  Aafke Land
  Marije Liscaljet (alumna)
  Eelco van Anken (alumnus)