The Molecular Architecture of Native BBSome Obtained by an Integrated Structural Approach

Abstract

Highlights d 4.9-Å density map of the intact native BBSome obtained by single-particle cryo-EM d Ca model generated using Rosetta establishes the subunit organization d BBSome subunits have a very high degree of interconnectivity d In solution, the BBSome exists predominantly in an autoinhibited state

Structure of the human BBSome core complex in the open conformation

“…Mutations at the BBS4-BBS1 and BBS8-BBS9 interfaces ( Table 1), such as Q325R 25 at the side of the BBS9 b-propeller ( Figure 4B) or E224K 30 and R268P 31 in the central cavity of the BBS1 b-propeller ( Figure 4C) lead to disease in patients, indicating that proper interaction between these subunits is crucial for a functional BBSome.…”

“…The GAE domain of BBS1 is also involved in interactions with BBS8. The glutamine residues 439 and 445 in BBS8 are located at this interface and their mutation results in disease (Q439H, Q445K) 25,[35][36][37] ( Figure 4A, Table 1).…”

“…While this manuscript was being prepared, the structure of the bovine BBSome complex at 4.9 Å was published, in which the Arl6 binding site is blocked by the subunits BBS2 and BBS7 25 . These two additional subunits form a highly intertwined lobe that contacts the BBSome core at the C-terminal hairpin in BBS9 and at the b-propeller of BBS1, thereby temporarily blocking the binding site for Arl6 25 . The overall architecture of the two complexes is very similar, but interestingly, in a rigid body overlay of our structure with the bovine BBSome, the Arl6 binding site does not clash with BBS2 or BBS7 ( Figure S6).…”

“…It is interesting to note that in native and heterologous expression, the top lobe formed by BBS2 and BBS7 readily dissociates 25 from the complex or renders it insoluble 26 . Likewise, BBS5 was present in substoichiometric amounts in both purifications.…”

“…The crystal structures of this domain in complex with Arl6 23 , as well as the b-propeller of BBS9 24 , provide the only currently available high-resolution structural information on BBSome subdomains. While this manuscript was in preparation, the medium-resolution structure of a BBSome complex purified from bovine retina was reported 25 . The structure revealed that BBS2 and BBS7 form a top lobe that blocks the Arl6 interaction site on BBS1.…”