Post‐translational modification directs nuclear and hyphal tip localization of Candida albicans mRNA‐binding protein Slr1

Abstract

Summary The morphological transition of the opportunistic fungal pathogen Candida albicans from budding to hyphal growth has been implicated in its ability to cause disease in animal models. Absence of SR-like RNA-binding protein Slr1 slows hyphal formation and decreases virulence in a systemic candidiasis model, suggesting a role for post-transcriptional regulation in these processes. SR (serine-arginine)-rich proteins influence multiple steps in mRNA metabolism and their localization and function are frequently controlled by modification. We now demonstrate that Slr1 binds to polyadenylated RNA and that its intracellular localization is modulated by phosphorylation and methylation. Wildtype Slr1-GFP is predominantly nuclear, but also co-fractionates with translating ribosomes. The non-phosphorylatable slr1-6SA-GFP protein, in which six serines in SR/RS clusters are substituted with alanines, primarily localizes to the cytoplasm in budding cells. Intriguingly, hyphal cells display an slr1-6SA-GFP focus at the tip near the Spitzenkörper, a vesicular structure involved in molecular trafficking to the tip. The presence of slr1-6SA-GFP hyphal tip foci is reduced in the absence of the mRNA-transport protein She3, suggesting that unphosphorylated Slr1 associates with mRNA-protein complexes transported to the tip. The impact of SLR1 deletion on hyphal formation and function thus may be partially due to a role in hyphal mRNA transport.

Fungal Morphogenesis, from the Polarized Growth of Hyphae to Complex Reproduction and Infection Structures

“…In addition to patches, F-actin cables are found along hyphae, probably serving as tracks for the transport of various cargoes, including secretory and endocytic vesicles, mRNAs, and Golgi equivalents, to the sites of growth and cell division, as shown in cells of S. cerevisiae and, more recently, in hyphae of Candida albicans (92,184,208,237,238). In some fungal species, these cables are arranged in the cytoplasm close to the cell cortex (57, 239).…”

Messenger RNA transport in the opportunistic fungal pathogen Candida albicans

“…Recent data indicate that the C. albicans S R- l ike R NA-binding protein Slr1, which has no apparent S. cerevisiae ortholog, may be a candidate to aid in mRNA transport to the hyphal tip (Ariyachet, et al 2017). Although Slr1 is a predominantly nuclear phosphoprotein, a fraction of Slr1 copurifies with 80S ribosomes and polysomes, suggesting a cytoplasmic role for Slr1.…”

“…Serine-to-alanine mutations in six SR/RS dipeptides at the C-terminus of Slr1 block phosphorylation of this slr1-6SA mutant protein and increase its cytoplasmic localization compared to wild type Slr1. Intriguingly, in hyphal cells the slr1-6SA-GFP mutant protein is often detected in a focus at the hyphal tip (Ariyachet, et al 2017). The slr1-6SA-GFP hyphal tip focus partially overlaps with the Spitzenkörper (Ariyachet, et al 2017) and is reminiscent of the concentration of Ca ASH1 and other CaShe3-transported mRNAs at the hyphal tip (Elson, et al 2009).…”

“…1b). In cells that lack CaShe3, however, hyphal tip localization of slr1-6SA-GFP decreases ~4 fold (Ariyachet, et al 2017), suggesting that Slr1 may travel to the tip in part with CaShe3 transport complexes. In addition, both Sc ASH1 mRNA in S. cerevisiae (Beach and Bloom 2001) and slr1-6SA-GFP in C. albicans (Ariyachet, et al 2017) appear in foci at the bud neck in large-budded cells.…”

“…In cells that lack CaShe3, however, hyphal tip localization of slr1-6SA-GFP decreases ~4 fold (Ariyachet, et al 2017), suggesting that Slr1 may travel to the tip in part with CaShe3 transport complexes. In addition, both Sc ASH1 mRNA in S. cerevisiae (Beach and Bloom 2001) and slr1-6SA-GFP in C. albicans (Ariyachet, et al 2017) appear in foci at the bud neck in large-budded cells. Future experiments to test binding of Slr1 to CaShe3 and CaShe3-transported mRNAs, as well as to determine the localization of these mRNAs in the absence of Slr1, will help define whether wildtype Slr1 plays a role in CaShe3-mediated mRNA transport in C. albicans .…”