Moonlighting Crypto-Enzymes and Domains as Ancient and Versatile Signaling Devices

Area of Science:

• Biochemistry
• Molecular Biology
• Enzymology

Background:

• Cryptic guanylate cyclases (GCs) and adenylate cyclases (ACs) are increasingly identified within complex proteins across prokaryotes and eukaryotes.
• These enzymes play roles in intramolecular regulation and signal generation within larger protein structures.

Purpose of the Study:

• To review the structural and functional characteristics of novel GCs and ACs.
• To explore their roles in regulating complex protein functions, such as the phytosulfokine receptor (PSKR).
• To investigate the potential of motif searches for discovering new cyclases and ancient biological functions.

Main Methods:

• Literature review of structural and functional data for GCs and ACs.
• Analysis of the Arabidopsis thaliana K+ uptake permease (AtKUP5) as a case study for multidomain enzyme architecture.
• Application of motif searches based on conserved amino acids in catalytic centers.

Main Results:

• Identified GCs and ACs regulate intramolecular functions and contribute to signal tuning.
• The AtKUP5 protein integrates K+ flux sensing and hormone responses with cAMP homeostasis via its AC and phosphodiesterase (PDE) sites.
• Motif searches successfully led to the discovery of GCs and ACs, demonstrating their utility.

Conclusions:

• Complex proteins harbor diverse, catalytically active GCs and ACs that are crucial for cellular regulation.
• Motif-based searches are a powerful strategy for discovering previously unknown enzyme active sites and ancient biological functions, including hormone and gas binding.
• This approach holds promise for expanding the known repertoire of enzymes in bacterial, fungal, and animal proteomes.