Biology subjects

Reales, E.

Publications and source records attributed to Reales, E..

3 recordsLinked to original sources

bioRxiv2017

The Habc Domain of Syntaxin 3 is a Ubiquitin Binding Domain with Specificity for K63-Linked Poly-Ubiquitin Chains

Syntaxins are a family of membrane-anchored SNARE proteins that are essential components required for membrane fusion in all eukaryotic intracellular membrane trafficking pathways. Syntaxins contain an N-terminal regulatory domain, termed the Habc domain, that is not highly conserved at the primary sequence level but folds into a three-helix bundle that is structurally conserved among family members. The syntaxin Habc domain has previously been found to be structurally very similar to the GAT domain present in GGA family members and related proteins that are otherwise completely unrelated to syntaxins. Because the GAT domain has been found to be a ubiquitin binding domain we hypothesized that the Habc domain of syntaxins may also bind to ubiquitin. Here, we report that the Habc domain of syntaxin 3 (Stx3) indeed binds to monomeric ubiquitin with low affinity. This domain binds efficiently to K63-linked poly-ubiquitin chains within a narrow range of chain lengths but not to K48-linked poly-ubiquitin chains. Other syntaxin family members also bind to K63-linked polyubiquitin chains but with different chain length specificities. Molecular modeling suggests that residues of the GGA3-GAT domain known to be important for ionic and hydrophobic interactions with ubiquitin have equivalent, conserved residues within the Habc domain of Stx3. We conclude that the syntaxin Habc domain and the GAT domain are both structurally and functionally related, and likely share a common ancestry despite sequence divergence. Binding of K63-linked poly-ubiquitin chains to the Habc domain may regulate the function of syntaxins in membrane fusion or may suggest additional functions of this protein family.

cell biology↗

bioRxiv2017

Mono-ubiquitination of syntaxin 3 leads to retrieval from the basolateral plasma membrane and facilitates cargo recruitment to exosomes

Syntaxin 3 (Stx3), a SNARE protein located and functioning at the apical plasma membrane of epithelial cells, is required for epithelial polarity. A fraction of Stx3 is localized to late endosomes / lysosomes though how it traffics there and its function in these organelles is unknown. Here we report that Stx3 undergoes mono - ubiquitination in a conserved polybasic domain. Stx3 present at the basolateral - but not the apical - plasma membrane is rapidly endocytosed, targeted to endosomes, internalized into intraluminal vesicles (ILVs) and excreted in exosomes. A non - ubiquitinatable mutant of Stx3 (Stx3 - 5R) fails to enter this pathway and leads to the inability of the apical exosomal cargo protein GPRC5B to enter the ILV / exosomal pathway. This suggests that ubiquitination of Stx3 leads to removal from the basolateral membrane to achieve apical polarity, that Stx3 plays a role in the recruitment of cargo to exosomes, and that the Stx3 - 5R mutant acts as a dominant - negative inhibitor. Human cytomegalovirus (HCMV) acquires its membrane in an intracellular compartment and we show that Stx3 - 5R strongly reduces the number of excreted infectious viral particles. Altogether these results suggest that Stx3 functions in the transport of specific proteins to apical exosomes and that HCMV exploit this pathway for virion excretion.

cell biology↗

bioRxiv2017

Soluble Syntaxin 3 Functions as a Transcription Regulator

Syntaxins - a conserved family of SNARE proteins - contain C-terminal transmembrane anchors required for their membrane fusion activity. Here we show that syntaxin 3 (Stx3) unexpectedly also functions as a nuclear regulator of gene expression. Alternative splicing leads to a soluble isoform, termed Stx3S, lacking the transmembrane anchor. Soluble Stx3S binds to the nuclear import factor RanBP5, targets to the nucleus and interacts physically and functionally with several transcription factors, including ETV4 and ATF2. Stx3S is differentially expressed in normal human tissues, during epithelial cell polarization, and in breast cancer vs. normal breast tissue. Inhibition of endogenous Stx3S expression leads to changes in the expression of cancer-associated genes and promotes cell proliferation. Similar nuclear-targeted, soluble forms of other syntaxins were identified suggesting that nuclear signaling is a conserved, novel function common among these membrane trafficking proteins.

cell biology↗