Keratin intermediate filaments form dynamic polymer networks that organize in specific ways dependent on the cell type, the stage of the cell cycle, and the state of the cell. In differentiated cells of the epidermis, they are organized by desmosomes, cell-cell adhesion complexes which provide essential mechanical integrity to this tissue. Despite this, we know little about how keratin organization is controlled and whether desmosomes locally regulate keratin dynamics in addition to binding pre-assembled filaments. Ndel1 is a desmosome-associated protein in the differentiated epidermis, though its function at these structures has not been examined. Here, we show that Ndel1 binds directly to keratin subunits through a motif conserved in all intermediate filament proteins. Further, Ndel1 was necessary for robust desmosome-keratin association and sufficient to reorganize keratins at distinct cellular sites. Lis1, a Ndel1 binding protein, was required for desmosomal localization of Ndel1, but not for its effects on keratin filaments. Finally, we use mouse genetics to demonstrate that loss of Ndel1 results in desmosome defects in the epidermis. Our data thus identifies Ndel1 as a desmosome-associated protein which promotes local assembly/reorganization of keratin filaments and is essential for robust desmosome formation.

Keratin intermediate filaments form dynamic polymer networks that organize in specific ways dependent on the cell type, the stage of the cell cycle, and the state of the cell. In differentiated cells of the epidermis, they are organized by desmosomes, cell-cell adhesion complexes which provide essential mechanical integrity to this tissue. Despite this, we know little about how keratin organization is controlled and whether desmosomes actively promote keratin assembly in addition to binding pre-assembled filaments. We recently discovered that Ndel1 is a desmosome-associated protein in differentiated epidermis. Here, we show that Ndel1 binds directly to keratin subunits through a motif conserved in all intermediate filament proteins.Further, Ndel1 is necessary for robust desmosome-keratin association and sufficient to reorganize keratins to distinct cellular sites. Lis1, a Ndel1 binding protein, is required for desmosomal localization of Ndel1, but not for its effects on keratin filaments. Finally, we use mouse genetics to demonstrate that loss of Ndel1 results in desmosome defects in the epidermis. Our data thus identify Ndel1 as a desmosome-associated protein which promotes local assembly/organization of keratin filaments and is essential for both robust cell adhesion and epidermal barrier function.

Dendritic cells (DC) are the 'sentinels' of the immune system and play an essential role in the presentation of antigenic material to T-cells. Human Immunodeficiency Virus 1 (HIV-1) is taken into DC evading degradation and transported to a virological synapse (VS) to promote Tcell infection. DC studies have led to the identification of several restriction factors that aid viral transmission, however how the virus is trafficked through the cell to the VS within DC is still unclear. A high-throughput siRNA method was developed in monocyte derived dendritic cells (MDDC) to identify membrane trafficking proteins involved in the transfer of HIV-1 from DC to T-cells. Characterisation of siRNA HITS showed that a functioning endolysosomal pathway is required for efficient HIV transfer to T-cells and trapping virus in early endosomalderived compartments significantly reduces viral trans-infection. Novel proteins identified by the siRNA screen share common roles in cargo sorting and transport from endosomal compartments and polarised transport to the plasma membrane. In addition, depletion of these target proteins resulted in an accumulation of virus in intracellular compartments and reduced VS formation between DC and T-cells. Identification of cellular candidates involved in viral transfer from DC to T-cells is critical for the development of potential drug therapies to combat HIV-1 infection in vivo.