Evolution of the Na-P(i) cotransport systems.

Area of Science:

• Molecular Evolution
• Membrane Transport Proteins
• Biochemistry

Background:

• Phosphate (P(i)) homeostasis is crucial and maintained by membrane transport systems.
• Two vertebrate Na-P(i) cotransporter families, NaPi-II and NaPi-III, share structural features with prokaryotic origins.
• NaPi-II and NaPi-III transporters exhibit conserved structural features, suggesting a prokaryotic origin and vertical evolutionary relationship.

Purpose of the Study:

• To investigate the molecular evolution of the NaPi-II protein family.
• To understand the evolutionary relationships between vertebrate and prokaryotic Na-P(i) transporters.
• To explore the origins and diversification of NaPi-II isoforms (NaPi-IIa and NaPi-IIb).

Main Methods:

• Comparative analysis of NaPi-II and NaPi-III transporter families across species.
• Expression of Vibrio cholerae NaPi-II homologue in Xenopus oocytes to assess P(i) transport function.
• RT-PCR with degenerate primers to identify NaPi-II-related mRNAs in various vertebrates.

Main Results:

• A functional P(i) transporter activity was confirmed for the V. cholerae NaPi-II homologue.
• NaPi-IIa and NaPi-IIb isoforms are responsible for regulated epithelial Na-dependent P(i) transport in vertebrates.
• Identification of NaPi-II-related mRNAs across diverse vertebrate species, suggesting ancient origins.

Conclusions:

• The NaPi-II protein family has undergone significant molecular evolution, including gene duplication events.
• A gene duplication event early in vertebrate evolution likely gave rise to NaPi-IIb and its paralogs.
• The emergence of the NaPi-IIa isoform is associated with the development of the mammalian nephron.