# Membrane proteins driving a cell-cell fusion reaction during fertilization

> **NIH NIH K99** · UNIV OF MARYLAND, COLLEGE PARK · 2022 · $100,000

## Abstract

Project Summary
 Membrane fusion between two gametes (e.g., sperm & egg) during fertilization is a crucial step in eukaryotic
life. In all organisms, the fusion reaction proceeds in two steps, membrane adhesion and bilayer merger.
Remarkably, for no single organism do we yet have the adhesion proteins for both gametes and the fusion
protein. Recently, our laboratory and others have shown that the ancient male gamete-specific protein HAP2 is
essential for fertilization across a broad range of eukaryotic taxa, including the pathogenic malaria organism
Plasmodium, green alga, ciliates, higher plants, and many metazoans. Our collaborative studies have also shown
that a key functional motif of Plasmodium HAP2 can be targeted for a transmission-blocking malaria vaccine.
Recent work demonstrated that HAP2 is structurally homologous to viral class II fusion proteins (e.g. Dengue and
Zika). Class II fusion proteins on enveloped viruses are triggered by the acidic environment of the endosome to
undergo a conformational reorganization from homo- or heterodimers into homotrimers that drive bilayer
merger and viral entry during infection. HAP2, however, is present at the cell surface and likely regulated
differently because it functions in a variety of milieus. Here, I propose to use fertilization in a bi-ciliated green
alga as a model system to investigate the mechanisms that regulate a eukaryotic class II fusion protein. For the
first time in any system, we now have identified the adhesion receptors on both gametes and the fusion protein.
The adhesion protein FUS1 on plus gametes and the adhesion protein MAR1 (which we have just identified) and
fusogen HAP2 on minus gametes. In what I feel is a major advance, I have also determined that MAR1 is
bifunctional. In addition to binding to FUS1, MAR1 is also biochemically and functionally associated with HAP2
on minus gametes. Moreover, we have determined that FUS1 and MAR1 dependent gamete adhesion is
necessary for the reconfiguration of HAP2 from its prefusion form on resting gametes into homotrimers that
drive membrane fusion. In this work, I intend to determine the pre-fusion conformation of HAP2 in resting minus
gametes, identify the domains of MAR1 and HAP2 that underlie their interactions in resting gametes, identify the
domains in FUS1 and MAR1 important for their binding during gametes interactions, and determine the changes
that MAR1 and HAP2 undergo during FUS1-MAR1 binding that activate HAP2 for fusion. The long-term
objectives of this proposal are to enhance our fundamental understanding of the mechanism of membrane fusion
at fertilization and at the same time provide new strategies for development of vaccines to target transmission
of pathogenic protozoa.

## Key facts

- **NIH application ID:** 10428846
- **Project number:** 1K99GM146112-01
- **Recipient organization:** UNIV OF MARYLAND, COLLEGE PARK
- **Principal Investigator:** Jennifer Fricke Pinello
- **Activity code:** K99 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2022
- **Award amount:** $100,000
- **Award type:** 1
- **Project period:** 2022-04-01 → 2024-03-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10428846

## Citation

> US National Institutes of Health, RePORTER application 10428846, Membrane proteins driving a cell-cell fusion reaction during fertilization (1K99GM146112-01). Retrieved via AI Analytics 2026-05-23 from https://api.ai-analytics.org/grant/nih/10428846. Licensed CC0.

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