Insights into the structure and evolution of the human SAGA complex by affinity-ligand purification

Area of Science:

• Molecular Biology
• Structural Biology
• Gene Regulation

Background:

• Human SAGA is a 20-subunit transcriptional coactivator with unique metazoan features.
• Metazoan SAGA incorporates a splicing-factor module (SPL) also found in U2 small nuclear ribonucleoprotein (U2snRNP).
• Gene duplication led to SAGA-specific paralogs (TAF5L, TAF6L), but their functional impact remains unclear.

Purpose of the Study:

• To determine the high-resolution structure of endogenous human SAGA.
• To elucidate the structural basis for the incorporation of the SPL module and TAF6L HEAT repeats into SAGA.
• To understand the functional consequences of SAGA-specific paralogs TAF5L and TAF6L.

Main Methods:

• Purification of endogenous human SAGA using an affinity ligand from undisturbed cells.
• High-resolution structural analysis of the SPL module and TAF6L HEAT repeat domain.
• Comparative structural analysis of SAGA and U2snRNP interactions.

Main Results:

• The study reveals the high-resolution structure of the SPL module and the TAF6L HEAT repeat domain within human SAGA.
• It elucidates the mechanism of SPL and HEAT repeat incorporation into the SAGA complex.
• Major structural differences in TAF6L/TAF5L paralogs were identified, enabling SPL accommodation.
• SPL interacts with SAGA via a smaller interface than with U2snRNP.

Conclusions:

• Human SAGA's structure accommodates the SPL module through specific interactions involving TAF6L HEAT repeats.
• The distinct interaction interface suggests SAGA may relay the SPL module to the splicing machinery.
• This structural insight clarifies the specialized role of metazoan SAGA and its paralogs in gene regulation and splicing.