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MBC in Press, published online ahead of print June 15, 2005
Mol. Biol. Cell 10.1091/mbc.E05-01-0028

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Submitted on January 18, 2005
Revised on April 27, 2005
Accepted on June 3, 2005

Genetic, Genomic, and Functional Analysis of the Granule Lattice Proteins in Tetrahymena Secretory Granules

Andrew T. Cowan,*{dagger} Grant R. Bowman,*{dagger} Kyle F. Edwards,* J. J. Emerson,{ddagger} and Aaron P. Turkewitz*

Departments of *Molecular Genetics and Cell Biology and {ddagger}Ecology and Evolution, The University of Chicago, Chicago IL 60637

Monitoring Editor: Randy Schekman

In some cells, the polypeptides stored in dense core secretory granules condense as ordered arrays. In ciliates like Tetrahymena thermophila, the resulting crystals function as projectiles, expanding upon exocytosis. Isolation of granule contents previously defined five Grl (Granule lattice) proteins as abundant core constituents, while a functional screen identified a 6th family member. We have now expanded this screen to identify the nonredundant components required for projectile assembly. The results, further supported by gene disruption experiments, indicate that six Grl proteins define the core structure. Both in vivo and in vitro data indicate that core assembly begins in the endoplasmic reticulum with formation of specific hetero-oligomeric Grl proprotein complexes. Four additional GRL-like genes were found in the T. thermophila genome. Grl2p and Grl6p are targeted to granules, but the transcripts are present at low levels and neither is essential for core assembly. The {Delta}GRL6 cells nonetheless showed a subtle change in granule morphology and a marked reduction in granule accumulation. Epistasis analysis suggests this results from accelerated loss of {Delta}GRL6 granules, rather than decreased synthesis. Our results provide insights into the organization of Grl-based granule cores, but also imply that the functions of Grl proteins extend beyond core assembly.

{dagger}These authors contributed equally to this work.

Address correspondence to: Aaron P. Turkewitz (apturkew{at}midway.uchicago.edu)






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