부산대학교 도서관

Parasitic protozoa of the genus Leishmania cycle between the phagolysosome of mammalian macrophages, where they reside as rounded intracellular amastigotes, and the midgut of female sand flies, which they colonize as elongated extracellular promastigotes. Previous studies indicated that protein kinase A (PKA) plays an important role in the initial steps of promastigote differentiation into amastigotes. Here, we describe a novel regulatory subunit of PKA (which we have named PKAR3) that is unique to Leishmania and most (but not all) other Kinetoplastidae. PKAR3 is localized to subpellicular microtubules (SPMT) in the cell cortex, where it recruits a specific catalytic subunit (PKAC3). Promastigotes of pkar3 or pkac3 null mutants lose their elongated shape and become rounded but remain flagellated. Truncation of an N-terminal formin homology (FH)-like domain of PKAR3 results in its detachment from the SPMT, also leading to rounded promastigotes. Thus, the tethering of PKAC3 via PKAR3 at the cell cortex is essential for maintenance of the elongated shape of promastigotes. This role of PKAR3 is reminiscent of PKARIβ and PKARIIβ binding to microtubules of mammalian neurons, which is essential for the elongation of dendrites and axons, respectively. Interestingly, PKAR3 binds nucleoside analogs, but not cAMP, with a high affinity similar to the PKAR1 isoform of Trypanosoma. We propose that these early-diverged protists have re-purposed PKA for a novel signaling pathway that spatiotemporally controls microtubule remodeling and cell shape. Author summary: Leishmania are single cell intracellular parasites of mammals, including humans. During their life, they cycle between macrophage phagolysosomes where they reside as rounded amastigotes, and the midgut of female sand flies, which they colonize as elongated promastigotes. Adaptation of Leishmania parasites to life in their mammalian host includes differentiation-associated changes in morphology from elongated promastigotes to rounded amastigotes. This study shows that a novel trypanosomatid-specific protein kinase A regulatory subunit (PKAR3) is essential for maintenance of the elongated shape of Leishmania promastigotes. PKAR3 is anchored to the subpellicular microtubule in the cell cortex by an N-terminal formin homology (FH)-like domain; where it recruits a developmentally regulated catalytic subunit (PKAC3). Exposure to the differentiation signal results in a rapid downregulation of PKA activity and subsequently cell rounding. Thus, the cAMP-independent PKAR3 appears to have been repurposed to serve an AKAP-like role in spatiotemporal control of morphogenesis, highlighting the evolutionary diversity of this family of signalling proteins. [ABSTRACT FROM AUTHOR]