Beyond a Linear Structure: The Tubular Organization of the Tripartite Attachment Complex and the Functional Role of TAC53

Abstract

The Tripartite Attachment Complex (TAC) is essential for mitochondrial DNA (kDNA) segregation in Trypanosoma brucei, providing a physical link between the flagellar basal body and the mitochondrial genome. Although the TACs hierarchical assembly and linear organization have been extensively studied, much remains to be discovered regarding its complete architecture and composition - for instance, our identification of a new TAC component underscores these knowledge gaps. Here, we use a combination of proteomics, RNA interference (RNAi), and Ultrastructure Expansion Microscopy (U-ExM) to characterize the TAC at high resolution and identify a novel component, TAC53 (Tb927.2.6100). Depletion of TAC53 in both procyclic and bloodstream forms results in kDNA missegregation and loss, a characteristic feature of TAC dysfunction. TAC53 localizes to the kDNA in a cell cycle-dependent manner and represents the most kDNA-proximal TAC component identified to date. U-ExM reveals a previously unrecognized tubular architecture of the TAC, with two distinct TAC structures per kDNA disc, suggesting a mechanism for precise kDNA alignment and segregation. Moreover, immunoprecipitation and imaging analyses indicate that TAC53 interacts with known TAC-associated proteins HMG44, KAP68, and KAP3, forming a network at TAC-kDNA the interface. These findings redefine our understanding of TAC architecture and function and identify TAC53 as a key structural component anchoring the mitochondrial genome in T. brucei. Significance StatementThis research identifies a new component (TAC53) of the tripartite attachment complex (TAC), a cellular machinery that anchors the mitochondrial DNA to a cytoskeletal structure, the basal body in Trypanosoma brucei. Using proteomics and high-resolution microscopy, we demonstrate that TAC53 is at the interface of the TAC and the mitochondrial DNA and likely the final piece in this structure. We also describe the overall tubular architecture of the TAC from the basal body to the mitochondrial matrix, and how the presence of two TAC structures per mitochondrial genome can explain the parasites ability to maintain its mitochondrial DNA accurately. In summary, we present a new component and the architecture of the currently best understood mitochondrial DNA segregation mechanism in biology.