TM9SF4 levels determine sorting of transmembrane domains in the early secretory pathway.

Previous studies have shown that TM9SF4 interacts with glycine-rich transmembrane domains (TMDs) and promotes their surface localization, presumably by escorting them along the secretory pathway. Here, we delineated the role of TM9 proteins in the sorting of TMDs. Our results indicate that TM9SF4 interacts with and sorts a variety of TMDs.

Role of the HIV-1 envelope transmembrane domain in intracellular sorting.

Background: The envelope protein of lentiviruses are type I transmembrane proteins, and their transmembrane domain contains conserved potentially charged residues. This highly unusual feature would be expected to cause endoplasmic reticulum (ER) localization. The aim of this study was to determine by which means the HIV-1 Env protein is transported to the cell surface although its transmembrane domain contains a conserved arginine residue.

MitoNEET-dependent formation of intermitochondrial junctions.

MitoNEET (mNEET) is a dimeric mitochondrial outer membrane protein implicated in many facets of human pathophysiology, notably diabetes and cancer, but its molecular function remains poorly characterized. In this study, we generated and analyzed mNEET KO cells and found that in these cells the mitochondrial network was disturbed. Analysis of 3D-EM reconstructions and of thin sections revealed that genetic inactivation of mNEET did not affect the size of mitochondria but that the frequency of intermitochondrial junctions was reduced.

TM9 family proteins control surface targeting of glycine-rich transmembrane domains.

TM9 family proteins (also named Phg1 proteins) have been previously shown to control cell adhesion by determining the cell surface localization of adhesion proteins such as the Dictyostelium SibA protein. Here, we show that the glycine-rich transmembrane domain (TMD) of SibA is sufficient to confer Phg1A-dependent surface targeting to a reporter protein. Accordingly, in Dictyostelium phg1A-knockout (KO) cells, proteins with glycine-rich TMDs were less efficiently transported out of the endoplasmic reticulum (ER) and to the cell surface.

Immunofluorescence labeling of cell surface antigens in Dictyostelium.

Background: Immunolocalization of cellular antigens typically requires fixation and permeabilization of cells, prior to incubation with antibodies.

Findings: Assessing a test protein abundantly present at the cell surface of Dictyostelium cells, we show that in fixed cells, permeabilization extracts almost completely this cell surface antigen. The extent of this artifact is variable depending on the procedure used for labeling and permeabilization, as well as on the antigen considered.