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. Author manuscript; available in PMC: 2016 Jun 1.
Published in final edited form as: Mol Microbiol. 2015 Mar 28;96(5):973–992. doi: 10.1111/mmi.12985

Figure 8. Proposed model for the influence of the ESCRT machinery on capsule formation and haem uptake.

Figure 8

In response to environmental or host signals, the core ESCRT machinery (ESCRT-I, II and III) participates with PKA to activate Rim101 and influence capsule attachment via regulation of cell wall composition. The bulk of the ESCRT influence on capsule formation occurs through activation of Rim101 given the shared defects in attachment. PKA may make a contribution to capsule formation in addition to its influence on Rim101 because expression of the N-terminal portion of Rim101 in double mutants defective for both the regulatory subunit of PKA and ESCRT functions (e.g., vps23 pkr1) did not restore capsule size to the level of the pkr1 mutant (Fig. 5). Additionally, the cAMP/PKA pathway is known to influence the transcription of genes for capsule synthesis as well as components of the secretory pathway (Pukkila-Worley et al., 2005; Hu et al., 2007). The ESCRT machinery contributes to iron acquisition from haem via Rim101 and Cig1-dependent and independent mechanisms. ESCRT mutants have a growth defect on haem and expression of the N-terminal portion of Rim101 in the mutants only slightly improved growth. The remaining growth defect may reflect a requirement for ESCRT participation in endocytosis of haem, and the minor improvement with activation of Rim101 may in part reflect the expression of Cig1 for haem uptake. The involvement of PKA is more complex because pka1 and pkr1 mutants do not show growth defects on haem (G. Hu, unpublished data).