Connecting the αα-hubs: same fold, disordered ligands, new functions

Area of Science:

• Structural biology
• Molecular biology
• Cellular signaling

Background:

• Signal fidelity relies on protein-protein interaction hubs that integrate information from complex interactomes.
• Alpha-alpha hubs (αα-hubs) are defined by a common secondary structure motif, comprising small α-helical domains within large, modular proteins that bind intrinsically disordered transcriptional regulators.

Purpose of the Study:

• To classify the harmonin-homology-domain (HHD) within the αα-hub family.
• To explore the expanded functions and novel features of αα-hubs upon inclusion of the HHD.

Main Methods:

• Comparative structural biology approaches were employed.
• Analysis of protein domains and their interactions.

Main Results:

• The harmonin-homology-domain (HHD), also known as the harmonin N-terminal domain (NTD), found in proteins like harmonin and whirlin, is assigned to the αα-hub class.
• This inclusion reveals that αα-hubs can scaffold supra-modular complexes involved in sensory perception, neurovascular integrity, and telomere regulation.
• A common characteristic is the binding of intrinsically disordered ligands with similar properties, enabling the integration of cellular cues without cross-talk.

Conclusions:

• The classification of HHD as an αα-hub unveils new functional aspects and highlights the benefit of grouping hub domains for cross-disciplinary discoveries.
• αα-hubs serve as valuable models for dissecting signal specificity and fidelity.
• These findings advance the understanding of hub proteins, cellular communication, and the role of intrinsically disordered proteins in signaling networks.