Genomic and Proteomic Evidence for a Second Family of Dense Core Granule Cargo Proteins in Tetrahymena thermophila

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Abstract
In addition to a family of structurally related proteins encoded by the Granule lattice (GRL) genes, the dense core granules in Tetrahymena thermophila contain a second, more heterogeneous family of proteins that can be defined by the presence of a domain homologous to β/γ‐crystallins. The founding members of the family, Induced during Granule Regeneration 1 (IGR1) and Granule Tip 1 (GRT1), were identified in previous screens for granule components. Analysis of the recently sequenced T. thermophila macronuclear genome has now uncovered 11 additional related genes. All family members have a single β/γ‐crystallin domain, but the overall predicted organization of family members is highly variable, and includes three other motifs that are conserved between subsets of family members. To demonstrate that these proteins are present within granules, polypeptides from a subcellular fraction enriched in granules were analyzed by mass spectrometry. This positively identified four of the predicted novel β/γ‐crystallin domain proteins. Both the functional evidence for IGR1 and GRT1 and the variability in the overall structure of this new protein family suggest that its members play roles that are distinct from those of the GRL family.