Protein phosphatase SppA regulates apical growth and dephosphorylates cell polarity determinant DivIVA inStreptomyces coelicolor
Authors: Passot Fanny M. , Cantlay Stuart , Flärdh Klas 
Published: March 2021

Bacteria that exhibit polar growth, i.e. build their peptidoglycan cell walls in restricted zones at cell poles, often show large morphological diversity and plasticity. However, their mechanisms for regulation of cell shape and cell wall assembly are poorly understood. The Gram-positiveStreptomycesbacteria, like other Actinobacteria, depend on the essential coiled coil protein DivIVA for establishment of cell polarity and direction of polar growth. Streptomycetes grow as filamentous hyphae that exhibit tip extension. New hyphal tips are generated by lateral branching. Cell shape is largely determined by the control of cell wall growth at these hyphal tips. The Ser/Thr protein kinase AfsK is involved in controlling polar growth and directly phosphorylates DivIVA. Here, we identify a protein phosphatase inStreptomyces coelicolor, SppA, that dephosphorylates DivIVAin vivoandin vitroand affects growth and cell shape. AnsppAmutant shows reduced rate of hyphal tip extension, altered hyphal branching patterns, and exhibits frequent spontaneous hyphal growth arrests, all contributing to the unusually dense mycelial structure and slow growth rate that characterizesppAmutants. These phenotypes are largely suppressed in anafsK sppAdouble mutant, showing that AfsK and SppA partially affect the same regulatory pathway and share target proteins that are involved control of polar growth inS. coelicolor. Strains with a non-phosphorylatable mutant DivIVA were constructed and confirm that the effect ofafsKon hyphal branching during normal growth is mediated by DivIVA phosphorylation. However, the phenotypic effects ofsppAdeletion are independent of DivIVA phosphorylation and must be mediated via other substrates. Altogether, this study identifies a PPP-family protein phosphatase directly involved in the control of polar growth and cell shape determination inS. coelicolorand underscore the importance of eukaryotic-type Ser/Thr phosphorylation in regulation of growth and cell envelope biogenesis in Actinobacteria.

Author summary

Gram-positive bacteria often use eukaryotic-like Ser/Thr protein phosphorylation in the regulation of central processes related to growth and cell cycle progression. Members of the Actinobacteria, including pathogenic mycobacteria and antibiotic-producing streptomycetes, exhibit a distinctive mode of polar growth, with cell wall synthesis being restricted to zones at cell poles and directed by the essential cell polarity determinant DivIVA. Previously, we have shown thatStreptomyces coelicolormodulates its machinery for polar growth and cell shape determination via the Ser/Thr kinase AfsK, which phosphorylates DivIVA. Here, we identify a phosphoprotein phosphatase that targets DivIVA and reverses the AfsK-mediated phosphorylation. We show that this phosphatase has strong effects on polar growth and cell shape, including hyphal branching and the rate of tip extension. The effects are to a large extent dependent on the kinase AfsK but do not require DivIVA phosphorylation as such. The results identify a new regulator of polar growth in streptomycetes, and highlight the importance of Ser/Thr phosphorylation in regulation of actinobacterial growth and cell shape determination.