# Structural and functional studies of axonemal microtubule inner proteins (MIPs)

> **NIH NIH R01** · WASHINGTON UNIVERSITY · 2021 · $368,550

## Abstract

Cilia (also known as flagella) are hair-like organelles protrude from the surface of most eukaryotic cells and are
responsible for cellular motility, fluid flow and sensory perception. A large group of human diseases, known as
ciliopathies, are caused by cilia dysfunction. The elongated shape of the cilium is supported by a highly
conserved structure called the axoneme. In most motile cilia, the axoneme has a 9+2 architecture in which
nine doublet microtubules (DMTs) surround a central pair of singlet microtubules (MTs). Bound periodically
along the length of each DMT are a variety of MT-associated proteins and complexes that decorate the
external and luminal surfaces with different periodicities (8,16, 24, 48 and 96-nm). These protein complexes
are found in coherent register along the entire length of the DMT, and loss of the coherence causes impaired
motility. How periodicity is established, maintained, and synchronized, especially over a long distance, has
been a long-standing question in the field. Each DMT has a distinctive structure with one complete ring of A-
tubule and one incomplete ring of B-tubule. How the unique architecture of the DMT is formed in vivo is still
unclear. Furthermore, many axonemal components are asymmetrically distributed in both the longitudinal
direction and the radial direction among the 9 DMTs. For example, 3 of the 9 DMTs contain a unique “beak”
structure in the proximal B-tubule lumens. To date, the molecular components and biological functions of the
beak are unknown. In this proposal, based on the identification of 33 microtubule inner proteins (MIPs) in our
recent work using high-resolution cryo-electron microscopy (cryo-EM), we propose to elucidate the functions of
individual MIPs during ciliogenesis using Chlamydomonas mutants. The objective of this application is to use a
combination of genetic and structural approaches to investigate the architectural principles governing the
assembly of DMTs and axonemes. We will focus on three key aspects of the architectural principles with the
following specific aims: (1) We will identify the key proteins responsible for maintaining coherent registry
between different periodicities, and investigate their mutual dependence, using Chlamydomonas mutants
lacking filamentous MIPs, external coiled-coil proteins, and proteins located at interfaces between different
repeat regions. (2) We will investigate the molecular mechanism governing B-tubule formation, and test two
hypotheses: (i) proteins located at the MT seam, the unique site within the A-tubule, are essential for B-tubule
formation; (ii) MIPs located at the outer junction (OJ) function to promote the B-tubule formation by shielding
the inhibitory effects of tubulin C-terminal tails at the OJ. (3): We will identify protein components of the beak
using high-resolution cryo-EM. Our preliminary data suggest that two main components are tektin filaments
and SAXO proteins. We will investigate their cellular functions and...

## Key facts

- **NIH application ID:** 10214643
- **Project number:** 5R01GM138854-02
- **Recipient organization:** WASHINGTON UNIVERSITY
- **Principal Investigator:** Rui Zhang
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2021
- **Award amount:** $368,550
- **Award type:** 5
- **Project period:** 2020-07-10 → 2025-06-30

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10214643

## Citation

> US National Institutes of Health, RePORTER application 10214643, Structural and functional studies of axonemal microtubule inner proteins (MIPs) (5R01GM138854-02). Retrieved via AI Analytics 2026-05-23 from https://api.ai-analytics.org/grant/nih/10214643. Licensed CC0.

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