TRIM16 homodimerizes through its coiled-coil domain

<p>. (A) TRIM16-GFP domain deletion plasmids. (B) Co-transfection of TRIM16-GFP and TRIM16-myc-His in HEK293 cells and subsequent immunoprecipitation by anti-myc antibody (Ab) and Western blot with anti-GFP antibody. Whole cell extract (WCE) used as total input. (C) TRIM16 homodimerizes through its coiled-coil domain. GFP deletion mutants were co-transfected with the TRIM16-myc-His vector. Anti-GFP antibody was used to pull down proteins binding the GFP tagged proteins and the TRIM16-myc-His was used to detect self-association via its different tag (right panel). Transfection efficiency was confirmed (left panel). TRIM16-GFP mutants were efficiently pulled down (middle panel). * non-specific bands, # refer to text.</p

Amino acid sequence comparison of TRIM16 and MID1 and modeling of TRIM16 B-boxes.

<p>The conserved residues in the zinc binding regions are in bold underlined type and are; cysteine; C, histidine; H, alanine; A aspartic acid; D. (<b>A</b>) TRIM16 and MID1 share the zinc binding consensus sequence for B-box1. (B) TRIM16 and MID1 share the Zinc binding consensus sequence for B-box2. (C/D) Modeling of B-boxes reveals zinc binding capability. Superimposition of the alpha-carbon backbone of the B-boxes from the MDM1 NMR structure (purple) and the homology model of TRIM16 (blue-grey). These two structures overlay with an average root-mean-square deviation of 0.4 Å.</p

B-boxes are required for TRIM16’s E3 ligase activity.

<p>(A) TRIM16 <i>in vivo</i> polyubiquitination assay. In HEK293 cells, HA-Ub was co-transfected with various TRIM16-GFP domain deletion expression plasmids. The protein lysate was subjected to immunoprecipitation by GFP antibody, and subjected to Western blot and probed with anti-HA antibody for Ub (right panel) and anti-GFP antibody for TRIM16 (left and middle panel). Two exposure times are shown. GFP antibodies detect both un-ubiquitinated and polyubiquitinated forms of TRIM16. Polyubiquitinated smear is present in the sample transfected with wild-type TRIM16 and shown by anti-GFP and anti-HA antibodies. (B) <i>In vitro</i> ubiquitination assay with myc-His tagged TRIM16 together with a panel of E2 enzymes, showing activity with the UbcH5 family. (C) <i>In vitro</i> ubiquitination assay with full-length TRIM16, TRIM16 domain deletion mutants or empty vector showing extensive polyubiquitination with full-length TRIM16 as detected by Western blot with anti-myc antibodies. Numbers indicate protein size in kDa. (D) Recombinant TRIM16 (Abnova) was evaluated for E3 activity in the presence of recombinant E1, UbcH5b, and HA-Ub as indicated. The capacity to catalyse auto-ubiquitination <i>in vitro</i> was observed only in the presence of ZnCl₂ and ATP. Western Blot (lower panel) with TRIM16 antibody showed amount of TRIM16 protein in each lane.</p

TRIM16 can heterodimerize with MID1, TRIM24 and PML.

<p>(A) Schematic structures of TRIM proteins used in heterodimerization studies. (B) TRIM16 binds MID1. Co-transfection of MID1-GFP and TRIM16-myc-His in HEK293 cells and subsequent immunoprecipitation by anti-myc antibody and Western blot with anti-GFP antibody. (C) MID1 was pulled down via its GFP antibody and a Western blot was performed to detect TRIM16-myc-His in the immunoprecipitated protein complex. (D) Whole cell lysates (WCL) of HEK293 cells transfected with empty vector (EV) or TRIM16-GFP were immunopreciptated with anti-GFP antibody. An anti-PML antibody was used to detect PML as a binding partner of TRIM16. (E) Lysates containing both TRIM16-GFP and TRIM24-His proteins were immunopreciptated with anti-GFP antibody. Anti-His antibody was used to detect the presence of TRIM24 in the TRIM16-associated complex.</p