Abstract
ABSTRACTThe generation of complex three-dimensional structures is a key developmental step for most eukaryotic organisms. The details of the molecular machinery controlling this step remain to be determined. An excellent model system to study this general process is the generation of three-dimensional fruiting bodies in filamentous fungi like Sordaria macrospora. Fruiting body development is controlled by subunits of the highly conserved striatin-interacting phosphatase and kinase (STRIPAK) complex, which has been described in organisms ranging from yeasts to humans. The highly conserved heterotrimeric protein phosphatase PP2A is a subunit of STRIPAK. Here, catalytic subunit 1 of PP2A was functionally characterized. The Δpp2Ac1 strain is sterile, unable to undergo hyphal fusion, and devoid of ascogonial septation. Further, PP2Ac1, together with STRIPAK subunit PRO22, governs vegetative and stress-related growth. We revealed in vitro catalytic activity of wild-type PP2Ac1, and our in vivo analysis showed that inactive PP2Ac1 blocks the complementation of the sterile deletion strain. Tandem affinity purification, followed by mass spectrometry and yeast two-hybrid analysis, verified that PP2Ac1 is a subunit of STRIPAK. Further, these data indicate links between the STRIPAK complex and other developmental signaling pathways, implying the presence of a large interconnected signaling network that controls eukaryotic developmental processes. The insights gained in our study can be transferred to higher eukaryotes and will be important for understanding eukaryotic cellular development in general.
Highlights
Reversible protein phosphorylations are among the most common posttranslational modifications [1] that are involved in various signaling pathways controlling a multitude of different processes, such as cell proliferation and sexual propagation [2, 3]
Previous tandem affinity purification (TAP)-mass spectrometry (MS) data obtained with PRO22 as bait supported the idea that PP2Ac1 is part of the fungal striatin-interacting phosphatase and kinase (STRIPAK) complex [17] and stimulated our interest in its functional characterization
This agrees with findings on other filamentous fungi, where homologs of PP2Ac1 are involved in hyphal fusion, vegetative growth, and sexual and asexual development [9, 18, 21]
Summary
Reversible protein phosphorylations are among the most common posttranslational modifications [1] that are involved in various signaling pathways controlling a multitude of different processes, such as cell proliferation and sexual propagation [2, 3]. PP2AA is encoded by a single gene, while two genes exist for PP2Ac isoforms that control developmental processes such as vegetative growth, hyphal fusion, virulence, and asexual and sexual propagation [8,9,10,11]. Regulatory PP2A subunits mediate substrate specificity and are classified into four families, called B, B=, B==, and B=== Members of these protein families within a single species show rather low sequence similarity, homologs of a given family are well. STRIPAK coordinates key cellular processes such as cytoskeleton organization, cell migration, and cell size, as well as morphology control, in mammalians [15] In fungi, this complex regulates sexual and asexual development, hyphal fusion, and virulence [14, 15]. Our studies indicate that catalytically active PP2Ac1 is a major regulator of fungal sexual development and must be considered a key component of the fungal STRIPAK complex
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