# Structural Basis for HIV-1 Gag assembly and Env incorporation

> **NIH NIH R37** · UNIVERSITY OF ALABAMA AT BIRMINGHAM · 2024 · $547,215

## Abstract

During the late phase of HIV-1 infection cycle, the virally encoded Gag polyproteins are targeted to the plasma
membrane (PM) for assembly, formation of immature particles, and virus release. Gag–PM binding is mediated
by interactions of the N-terminally myristoylated matrix (MA) domain with phosphatidylinositol 4,5-bisphosphate
(PI(4,5)P2). Concurrent to Gag assembly, the envelope (Env) protein is recruited to the PM for incorporation into
virus particles. Env recruitment, and hence gp41, to a nascent virion is essential for downstream infectivity.
Without gp41, there is no fusion and no infectivity. Several lines of evidence suggest that Env incorporation is
mediated by interactions between the cytoplasmic tail of gp41 (gp41CT) and the MA domain of Gag. It has been
long recognized that only a few (< 10) gp41 molecules are embedded in the MA layer. Both gp41CT and a well-
formed MA lattice are essential for incorporation and infectivity. It appears that it is not sufficient to only embed
gp41CT in the MA layer, but it is necessary for the MA layer to undergo a cleavage induced maturation step for
gp41 to become fully active. The incorporation and activation of gp41 has been a long-standing problem whose
solution requires a structural approach. Recent low-resolution cryo-electron tomography (cryo-ET) studies
proposed a model in which the MA domain undergoes a structural transformation to form distinct MA lattices
during assembly and upon maturation. During the current funding period, our lab has shown that MA forms a
hexamer of trimers lattice with a central hole, thought to accommodate gp41CT to promote incorporation into
virions. We have also shown that PI(4,5)P2 is capable of binding to alternate sites on MA, consistent with a novel
and perhaps distinct MA–membrane binding mechanisms during assembly of the immature particle and upon
maturation. However, the structural details of the MA lattice (immature and mature) bound to membrane, factors
that govern the MA conformational switch, factors that (de)stabilize the MA lattice, and the structural basis for
MA–gp41CT interaction during assembly and upon maturation, are still lacking. The aims of this proposal are
designed to elucidate the molecular mechanisms that render HIV infectious by studying how gp41CT is
embedded in the MA layer. We devised innovative “controlled assembly” approaches which will enable us to
generate biologically authentic substructures of the immature and mature gp41CT–MA complex. To study these
substructures, we have developed cryo-electron microscopy (cryo-EM) approaches which will allow us to use
single particle rather than cryo-ET techniques to enable near atomic structural determination. Our aims are to
(1) determine the structural basis for MA lattice formation during assembly of the immature particle and upon
maturation, (2) determine factors critical for MA lattice formation and Env incorporation, and to (3) determine the
structure of the MA–gp41CT–membrane complex ...

## Key facts

- **NIH application ID:** 10874679
- **Project number:** 5R37AI150901-15
- **Recipient organization:** UNIVERSITY OF ALABAMA AT BIRMINGHAM
- **Principal Investigator:** Jamil Subhi Saad
- **Activity code:** R37 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2024
- **Award amount:** $547,215
- **Award type:** 5
- **Project period:** 2010-05-15 → 2028-08-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10874679

## Citation

> US National Institutes of Health, RePORTER application 10874679, Structural Basis for HIV-1 Gag assembly and Env incorporation (5R37AI150901-15). Retrieved via AI Analytics 2026-05-24 from https://api.ai-analytics.org/grant/nih/10874679. Licensed CC0.

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