Independent Transport and Sorting of Functionally Distinct Protein Families in Tetrahymena thermophila Dense Core Secretory Granules

Abstract

Dense core granules (DCGs) in Tetrahymena thermophila contain two protein classes. Proteins in the first class, called granule lattice (GrI), coassemble to form a crystalline lattice within the granule lumen. Lattice expansion acts as a propulsive mechanism during DCG release, and GrI proteins are essential for efficient exocytosis. The second protein class, defined by a C-terminal β/γ-crystallin domain, is poorly understood. Here, we have analyzed the function and sorting of Grt1p (granule tip), which was previously identified as an abundant protein in this family. Cells lacking all copies of GRT1, together with the closely related GRT2, accumulate wild-type levels of docked DCGs. Unlike cells disrupted in any of the major GRLgenes, δGRT1 δGRT2 cells show no defect in secretion, indicating that neither exocytic fusion nor core expansion depends on GRT1. These results suggest that GrI protein sorting to DCGs is independent of Grt proteins. Consistent with this, the granule core lattice in XGRT1 XGRT2 cells appears identical to that in wild-type cells by electron microscopy, and the only biochemical component visibly absent is Grt1p itself. Moreover, gel filtration showed that GrI and Grt proteins in cell homogenates exist in nonoverlapping complexes, and affinity-isolated Grt1p complexes do not contain GrI proteins. These data demonstrate that two major classes of proteins in Tetrahymena DCGs are likely to be independently transported during DCG biosynthesis and play distinct roles in granule function. The role of Grt1p may primarily be postexocytic; consistent with this idea, DCG contents from XGRT1 XGRT2 cells appear less adhesive than those from the wild type. Copyright © 2009, American Society for Microbiology. All Rights Reserved.