# Conserved mechanisms of ciliary signaling and cell-cell fusion

> **NIH NIH R01** · UNIV OF MARYLAND, COLLEGE PARK · 2022 · $558,000

## Abstract

We propose to use fertilization in the green alga Chlamydomonas as a model system to investigate conserved
but still poorly understood cellular and molecular mechanisms of ciliary signaling and the gamete membrane
fusion reaction. When Chlamydomonas gametes of opposite mating types are mixed together they adhere to
each other by complementary adhesion receptors on their cilia. Interactions between the adhesion receptors
SAG1 on plus gametes and SAD1 on minus gametes initiate a signaling pathway that is transmitted to the cell
body and triggers a rapid increase in cellular cAMP that activates the gamete for cell-cell fusion. Regulation of
ciiliary cyclic nucleotides by ciliary receptor engagement has been conserved throughout evolution and in almost
all ciliary signaling events in vertebrates, the primary response to engagement of ciliary receptor is a change in
the concentration of cAMP within the organelles. Moreover, in most vertebrate ciliary systems, the mechanisms
that transmit the increase in ciliary cAMP to a cellular response are unclear. We have discovered that a protein
kinase, GSPK, in Chlamydomonas functions at a previously unrecognized step in ciliary signaling. GSPK is
localized in the cytoplasm, not the cilia, yet the entire cellular complement of GSPK is phosphorylated within 1
minute after engagement of the ciliary adhesion receptors. Furthermore, GSPK phosphorylation is upstream of
the cAMP increase and GSPK function is required for the rapid increase in cAMP. Our findings set the stage for
new strategies to investigate cellular and molecular mechanisms that couple ciliary receptor engagement to a
cytoplasmic response. We will use a combination of cell biological, biochemical, and imaging strategies to
investigate the mechanisms underlying GSPK function. Gamete activation also activates erection of apically
localized protrusions, the plus and minus mating structures, from the plasma membrane of each gamete that
are the sites of membrane fusion during fertilization..We have made exciting advances in understanding
conserved molecular mechanisms of the gamete membrane fusion reaction, and we now know more about cell-
cell fusion in Chlamydomonas than is known in any other system. 1) We showed that bilayer merger of the two
cells is driven by trimer formation of eukaryotic class II fusion protein, HAP2, a ancient protein required for
fertilization in organisms across kingdoms. 2) We have now identified the separate cell-cell adhesion proteins s
FUS1 and MAR1 at the fusion site of each cell. 3) We demonstrated that the adhesion protein MAR1 on minus
gametes not only binds to FUS1 on plus gametes, but MAR1 is also biochemically and functionally linked to
HAP2. 4) We showed that membrane adhesion mediated by FUS1-MAR1 interactions releases the restrained,
prefusion form of HAP2, leading to the irreversible structural rearrangements into the fusion-driving HAP2 trimer.
5) In collaborative laboratory studies and field studies in Africa, ...

## Key facts

- **NIH application ID:** 10522540
- **Project number:** 1R01GM146162-01A1
- **Recipient organization:** UNIV OF MARYLAND, COLLEGE PARK
- **Principal Investigator:** William J Snell
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2022
- **Award amount:** $558,000
- **Award type:** 1
- **Project period:** 2022-09-20 → 2026-06-30

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10522540

## Citation

> US National Institutes of Health, RePORTER application 10522540, Conserved mechanisms of ciliary signaling and cell-cell fusion (1R01GM146162-01A1). Retrieved via AI Analytics 2026-05-24 from https://api.ai-analytics.org/grant/nih/10522540. Licensed CC0.

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