Alpha solenoid

[12] Due to their propensity for forming large interaction surfaces well-suited to protein-protein interactions, and their flexible surfaces permitting binding of various cargo molecules, alpha solenoid proteins commonly function as transport proteins, particularly in transport between the nucleus and the cytoplasm.

[13] Transporters of other molecules, such as RNA, can also be of alpha solenoid architecture, as in exportin-5[14] or pentatricopeptide-repeat-containing RNA-binding proteins, which are particularly common in plants.

For example, regulatory subunit A (also known as PR65) of protein phosphatase 2A is a HEAT-repeat alpha solenoid whose conformational flexibility regulates access to the enzyme binding site.

[2] Evolutionary relationships between different alpha solenoid proteins are difficult to trace due to the low sequence homology of the repeats.

Most distinctively, a shared domain architecture consisting of an N-terminal beta propeller and a C-terminal alpha solenoid has been detected in both NPC and coat proteins, suggesting a possible common origin.

[10] The PVC superphylum is known for containing bacteria with unusually complex membrane morphology, and this discovery has been cited as evidence in favor of these organisms' status as an intermediate form between prokaryotes and eukaryotes.

The planctomycete Gemmata obscuriglobus has exceptionally complex membrane architecture and has been a source of controversy in the literature regarding the possibility that it has a membrane-bound "nucleoid" compartment enclosing its DNA.

[23][24][25][26][27][28] The identification of proteins with sequence similarities to HEAT repeats in the G. obscuriglobus proteome has been interpreted as support for the membrane-bound nucleoid hypothesis;[29] however, this has been disputed.

A large number of different computational methods have been developed to identify candidate alpha solenoid proteins based on their amino acid sequence.

An example of an alpha solenoid structure composed of 15 HEAT repeats . The protein phosphatase 2A regulatory subunit is shown with the N-terminus in blue at bottom and the C-terminus in red at top. A single helix-turn-helix motif is shown in the center with the outer helix in pink, the inner helix in green, and the turn in white. From PDB : 2IAE ​. [ 1 ]
The structure of the clathrin heavy chain leg segment showing helical repeats, with the N-terminus in blue at left and the C-terminus in red at right. [ 11 ]
The assembled heterotrimer of protein phosphatase 2A . Subunit A, consisting of 15 HEAT repeats, is shown in rainbow color with the N-terminus in blue at bottom and the C-terminus in red at top. The regulatory subunit B, consisting of irregular pseudo-HEAT repeats, is shown in light blue. The catalytic subunit C is shown in tan. (All from PDB : 2IAE ​.) Superposed is the unbound form of the regulatory subunit B in gray (from PDB : 1B3U ​), illustrating the flexibility of this alpha solenoid protein. Conformational changes in HEAT repeat 11 result in flexing the C-terminal end of the protein to accommodate binding of the catalytic subunit. [ 1 ] [ 17 ]