Two homologous rat cellular retinol-binding proteins differ in local conformational flexibility

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Dynamics

Background:

• Cellular retinol-binding proteins I (CRBP I) and II (CRBP II) are homologous proteins crucial for vitamin A homeostasis.
• Distinct roles of CRBP I and CRBP II in retinoid metabolism are suggested but not fully understood at a molecular level.

Purpose of the Study:

• To compare the solution structure and dynamics of CRBP I and CRBP II using multidimensional NMR.
• To elucidate the molecular basis for the distinct functions of CRBP I and CRBP II in vitamin A metabolism.

Main Methods:

• Multidimensional NMR spectroscopy (including NOESY and 15N relaxation dispersion experiments).
• Lipari-Szabo formalism for analyzing conformational rigidity.
• Analysis of conformational exchange on micro- to millisecond timescales.

Main Results:

• Structural differences between CRBP I and CRBP II are localized to the alphaII helix.
• Retinol binding dynamics differ between CRBP I and CRBP II, evidenced by NOE cross-peaks.
• Retinol binding significantly reduces conformational flexibility on the micro- to millisecond timescale for both proteins.
• Apo-CRBP II exhibits increased chemical exchange rates and chemical shift differences in the portal region compared to apo-CRBP I.

Conclusions:

• Differences in conformational flexibility, especially in the alphaII helix and portal region, likely dictate the distinct interactions of CRBP I and CRBP II with ligands, membranes, and enzymes.
• These findings provide molecular insights into the differential roles of CRBP I and CRBP II in vitamin A homeostasis.