A09: Dynamic endosomal transport and unconventional secretion

In polarized cells vesicular long-distance transport is essential for efficient growth and intercellular communication. In fungi, endosomes function as such versatile membranous carriers. We recently discovered that active mRNA transport along microtubules is crucial for unipolar growth and secretion of endochitinase Cts1 in infectious hyphae of the plant pathogenic fungus Ustilago maydis. The underlying mechanism is co-transport of mRNAs on specific endosomes, designated early endosomes, which have been described to mature into late endosomes in other organisms. However, in U. maydis hyphae they retain their identity and form a defined endosomal compartment. These highly dynamic endosomes are not only used to recycle transmembrane proteins but also to deliver newly synthesized proteins to the hyphal tip hereby sustaining efficient polar growth and secretion of Cts1. Surprisingly, the latter follows an unconventional route independent of canonical ER-mediated secretion. Such alternative processes are known from other organisms including yeast and animal cells, however, at present only few mechanisms are described in detail. Here we plan to address the following key questions: (i) How are mRNAs coupled to shuttling endosomes and which factors regulate the identity of these specific endosomes? (ii) What is the molecular mechanism of unconventional Cts1 secretion and is this mechanism evolutionarily conserved? Results will provide new insights in cellular processes linking spatio-temporal regulation of dynamic intracellular membrane trafficking with extracellular functions involving extensive protein export.

Figure 1: Model depicting endosomal mRNA transport in U. maydis. (A) Schematic of an infectious hypha with retraction septa and cytoplasm-free section at the left. Endosomes shuttle along antiparallel microtubule bundles. Plus-end directed transport is mediated by Kin3, whereas dynein transports endosomes towards minus ends. Cellular functions in anterograde and retrograde direction are indicated. (B) Close up of endosome shown above. Endosomal components and their cellular functions are indicated. Rrm4 mediates local translation to load the cytoplasmic surface of endosomes with septins for assembly and transport. Question marks highlight knowledge gaps, which this proposal aims to close.

Project leaders:
Dr. Kerstin Schipper, undefined email, and Prof. Dr. Michael Feldbrügge, undefined email,
                          undefinedInstitute of Microbiology

Researchers: Kira Müntjes, undefined email 
                        Michèle Reindl, undefined email
                        Senthil Devan, undefined email

Responsible for the content: E-MailRedaktionsteam SFB 1208