Setdb1 and Atf7IP form a hetero-trimeric complex that blocks Setdb1 nuclear export.

Histone H3K9 methylation (H3K9me) by Setdb1 silences retrotransposons (rTEs) by sequestering them in heterochromatin. Atf7IP is a constitutive binding partner of Setdb1 and is responsible for Setdb1 nuclear localization, activation, and chromatin recruitment. However, structural details of the Setdb1/Atf7IP interaction have not been elucidated. We used Alphafold2 predictions and biochemical reconstitutions to show that one copy of Setdb1 and two copies of Atf7IP form a hetero-trimeric complex in vitro and in cells. We also find that Atf7IP self-associates, forming multimeric complexes that are resolved upon Setdb1 binding. Setdb1 binds to Atf7IP through coiled coil interactions that include both Setdb1 nuclear export signals (NES). Atf7IP directly competes with Crm1 to bind the Setdb1 NES motifs, explaining how Atf7IP prevents Crm1-mediated nuclear export of Setdb1. Setdb1 also forms hetero-trimeric complexes with the Atf7IP paralog Atf7IP2, and we show that Setdb1 can form mixed heterotrimers comprising one copy of each Setdb1, Atf7IP, and Atf7IP2. Atf7IP and Atf7IP2 are co-expressed in many tissues, suggesting that heterotrimers with different compositions of Atf7IP and Atf7IP2 may differentially regulate H3K9me by fine-tuning Setdb1 localization and activity.