Injection of poly(β‐l‐malate) into the plasmodium of Physarum polycephalum shortens the cell cycle and increases the growth rate

Abstract

Poly(b-L-malate) (PMLA) has been reported as an unconventional, physiologically important biopolymer in plasmodia of myxomycetes, and has been proposed to function in the storage and transport of nuclear proteins by mimicking the phospho(deoxy)ribose backbone of nucleic acids. It is distributed in the cytoplasm and especially in the nuclei of these giant, multinucleate cells. We report here for the first time an increase in growth rate and a shortening of the cell cycle after the injection of purified PMLA. By comparing two strains of Physarum polycephalum that differed in their production levels of PMLA, it was found that growth activation and cell cycle shortening correlated with the relative increases of PMLA levels in the cytoplasm or the nuclei. Growth rates of a low PMLA producer strain (LU897 · LU898) were increased by 40-50% while those of a high producer strain (M 3 CVIII) were increased by only 0-17% in comparison with controls. In both strains, shortening of the cell cycle occurred to a similar extent (7.2-9.5%), and this was associated with similar increases in nuclear PMLA levels. The effects showed saturation dependences with regard to the amount of injected PMLA. A steep rise of intracellular PMLA shortly after injection was followed by the appearance of histone H1 in the cytoplasm. The increase in growth rate, the shortening of the cell cycle duration and the appearance of H1 in the cytoplasm suggest that PMLA competes with nucleic acids in binding to proteins that control translation and/or transcription. Thus, PMLA could play an important role in the coordination of molecular pathways that are responsible for the synchronous functioning of the multinucleate plasmodium. Keywords 1: cell cycle; growth rate; Physarum polycephalum; plasmodium; polymalic acid.In the absence of cytokinesis, repeated nuclear divisions give rise to giant multinucleate cells (plasmodia) in Physarum polycephalum [1], a well studied representative of the myxomycete family. One of the notable features of plasmodia is the high synchrony of events during the cell cycle. The maintenance of this synchrony over large cellular distances must require an activity that accounts for the rapid and ubiquitous distribution and coordination of protein activities in the periodical cell cycle events. We have previously identified the unusual polyanion poly(b-L-malate) (PMLA) as a specific component of the plasmodium that fulfils the requirements for such a Ôdistributing activityÕ [2,3]. Its level in the nuclei is kept constant by constitutive synthesis and secretion of excess polymer from the cytoplasm to the culture medium, and the levels in the nuclei for different strains are of the same magnitude [4]. PMLA binds reversibly to histones, DNA polymerases, and other DNAinteracting proteins, thus favouring the formation of large complexes consisting of a variety of proteins. The binding involves specifically the array of negative carboxylates on the PMLA chain that is isosteric with the array of phosphates in nucleic acids [2,[5][6][7][8][...

Identification of substrates for transglutaminase in Physarum polycephalum, an acellular slime mold, upon cellular mechanical damage

“…The plasmodium, used in this study, is a giant and multinucleated cell with a veined structure and no internal cell walls. So far, Physarum has been used mainly in studies on the cell cycle, inheritance of mitochondrial DNA, and cytoplasmic streaming [13–18]. Physarum is also an appropriate model organism for studies on responses to environmental stress.…”

Effects of medium composition on the growth and lipid production of microplasmodia of Physarum polycephalum

“…The microplasmodial culture appeared light yellow. It has been reported that for many strains of P. polycephalum , the color of microplasmodia is mostly yellow, such as M 3 CVII and M 3 CVIII, except for two albino strains, including LU897 × LU898 and LU887, which have a white color . The microplasmodia were of different sizes, ranging from 50 to 2,000 μm in diameter.…”

“…It has been reported that for many strains of P. polycephalum, the color of microplasmodia is mostly yellow, such as M 3 CVII and M 3 CVIII, except for two albino strains, including LU897 × LU898 and LU887, which have a white color. 15 The microplasmodia were of different sizes, ranging from 50 to 2,000 μm in diameter. The resulting sizes were somewhat larger than reported in previous studies, in which microplasmodial size varied from 50 to 1,000 μm.…”

Stage specific expression of poly(malic acid)‐affiliated genes in the life cycle of Physarum polycephalum

“…The other tube was kept on ice and was used to measure the amount of endogenous malate. Polymalic acid was assessed on basis of the malate dehydrogenase reaction as described [8]. Protein was assayed according to the method of Bradford [38].…”

“…Due to its structural similarity to the backbone of nucleic acids, PMLA has been proposed to bind nuclear proteins and function in a molecular transporter system ([6] and references therein). Injection of PMLA into plasmodia increased the growth rate and shortened cell cycle duration, indicating that it could also be involved in the molecular events responsible for the synchronization of events in the plasmodium [7,8]. PMLA is synthesized from l ‐malate derived from d ‐glucose through the glycolytic pathway and the tricarboxylic acid cycle [9].…”