Lam6 Is Important for Cross-Talk between Contact Sites
(A) Fluorescent microscopy shows that the ERMES contact (as measured by the number of Mdm34-GFP puncta per cell) expanded in the Δvps39 background relative to WT. However, the expansion did not occur on the background of Δvps39 Δlam6, demonstrating that Lam6 is necessary for ERMES expansion under these conditions. Scale bar represents 5 μm.
(B) Quantitation of (A). Bars represent the percentage of cells containing the specific number of puncta/cell out of total cells counted for the strain (for WT, n = 234 cells; for Δvps39, n = 246; for Δvps39 Δlam6, n = 271).
(C) Fluorescent microscopy demonstrates that downregulating ERMES contacts (by growing GALp-MDM34 strains in glucose) indeed caused expansion of vCLAMP (GFP-Vps39). However, this expansion was diminished in a Δlam6 background. Scale bar represents 5 μm (see also ).
(D) A model summarizing our hypothesis on the way that Lam6 functions to regulate contact site communication. In normal cells, where all contact sites are intact, Lam6 is found mostly in ERMES contact sites, and to a lesser extent in vCLAMP and NVJ. However, in case a contact site is lost, the Lam6 proteins that were localized to this contact become free to associate with the other contact sites, thus signaling that they must expand. Raising Lam6 levels in any other way in a contact site therefore would cause an expansion of this contact site.