Regulation of levels of actin threonine phosphorylation during life cycle of Physarum polycephalum

Abstract

Under various environmental stresses, the true slime mold Physarum polycephalum converts into dormant forms, such as microcysts, sclerotia, and spores, which can survive in adverse environments for a considerable period of time. In drought-induced sclerotia, actin is threonine phosphorylated, which blocks its ability to polymerize into filaments. It is known that fragmin and actin-fragmin kinase (AFK) mediate this phosphorylation event. In this work, we demonstrate that high levels of actin threonine phosphorylation are also found in other dormant cells, including microcysts and spores. As the threonine phosphorylation of actin in microcysts and sclerotia were induced by drought stress but not by other stresses, we suggest that drought stress is essential for actin phosphorylation in both cell types. Although characteristic filamentous actin structures (dot- or rod-like structures) were observed in microcysts, sclerotia, and spores, actin phosphorylation was not required for the formation of these structures. Prior to the formation of both microcysts and sclerotia, AFK mRNA expression was activated transiently, whereas fragmin mRNA levels decreased. Our results suggest that drought stress and AFK might be involved in the threonine phosphorylation of actin.

Transcriptomic changes arising during light-induced sporulation in Physarum polycephalum

“…Physarum cell types differ in actin organization but express the same actin genes, suggesting that changes in actin-binding proteins are responsible for the differences in actin organization [ 28 ]. Physarum possesses several classes of actin- binding proteins, and most of these proteins display cell type-specific patterns of expression, but their precise roles are not known [ 29 , 30 ]. Nevertheless, expression changes in genes coding for actin-binding proteins correlate with modifications in cell organization and behavior [ 27 ].…”

“…In addition, we noticed that transcripts coding for Physarum plasmodia-specific actin-binding proteins, such as profilin P (PROP) [ 30 ] and fragmin P (FRGP) [ 36 ], are downregulated after photoinduction (Figure 3 ). FRGP enables actin phosphorylation by the actin-fragmin kinase (AFK), and binds phosphorylated actin [ 29 , 36 ]. Therefore it is possible that during sporulation these proteins are involved in the reorganization of the subcellular compartments via interactions with the actin cytoskeleton.…”

Fetal calf serum heat inactivation and lipopolysaccharide contamination influence the human T lymphoblast proteome and phosphoproteome

“…Two cytoskeletal proteins ACTA and ACTB (spot 13 and 17 respectively) displayed increase phosphorylation signals in the HE group, as compared to NHE group. These proteins are ubiquitously expressed in eukaryotic cells, are involved in the cytoskeletal architecture of the cell [48], and they are modified by phosphorylation during stressful conditions [49]. The ICLN protein (spot 18) participates in the regulation of small nuclear ribonucleoproteins, (snRNP) biogenesis, and is an essential component of spliceosomes [50].…”