Examining potential roles of protein phosphatase 5 by identification of protein binding partners and characterization of its yeast homologue, PPT1

Jee-Yeong Jeong, Purdue University

Abstract

Protein phosphatase 5 (PP5) contains a C-terminal catalytic domain that is structurally related to the catalytic subunits of members of PPP serine/threonine phosphatase family and a unique N-terminal domain consisting of three tetratricopeptide repeats (TPRs). Although the physiological roles of PP5 have not been well established, evidence implicates PP5 in a variety of cellular events, including cell proliferation, signal transduction, and stress response. The purpose of the present study was to understand the physiological functions and regulation of PP5. To achieve this objective, two specific strategies were employed. First, a homologue of PP5 from the yeast Saccharomyces cerevisiae , protein phosphatase T1 (PPT1), was characterized. Although PPT1 and human PP5 exhibit 42% sequence identity, the relationship between these two enzymes has not been explored at biochemical or cellular levels. PPT1 exhibited similar control activity as PP5. PPT1 was stimulated by arachidonic acid (AA) toward 32P-MBP like PP5. However, in contrast to PP5, PPT1 activity was not activated by AA toward 32P-casein. PPT1 was localized throughout the cell. PPT1 expression peaked in the early log phase of growth at both the mRNA and protein level. This is consistent with the findings for PP5 homologues in other species. This study suggests that PPT1 may be the functional homologue of PP5 but may exhibit differences in substrate preference and the mode of regulation. Further investigation of PPT1 in yeast may identify functions of PP5 in higher organisms. Second, proteins that potentially interact with PPS were identified by yeast two-hybrid screens using the TPR domain as bait. A known PP5 binding protein, Hsp90 was identified in the screens, supporting the possibility that new candidates including BRL, LZTR-1, and three unknown proteins may also be physiologically relevant interacting partners. This study suggests potential roles of PP5 in controlling transcriptional regulators and the cytoskeleton, based on the putative functions of the identified proteins. The physiological relevance and significance of the interactions will be further investigated to understand the functions and regulation of PP5.

Degree

Ph.D.

Advisors

Rossie, Purdue University.

Subject Area

Molecular biology|Biochemistry

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