Calcium-dependent changes in structure of calmodulin with substance P

Area of Science:

• Biochemistry
• Structural Biology
• Neuroscience

Background:

• Calmodulin (CaM) is a crucial calcium-binding protein involved in numerous cellular processes.
• Substance P is a neuropeptide that plays a role in neurotransmission and pain signaling.
• Understanding the structural dynamics of CaM-peptide complexes is vital for deciphering cellular signaling pathways.

Purpose of the Study:

• To investigate the solution structure of calmodulin complexed with substance P using synchrotron radiation.
• To elucidate the role of calcium ions (Ca2+) in the structural changes of the calmodulin-substance P complex.
• To examine the biochemical properties of the complex, including its susceptibility to proteolysis.

Main Methods:

• Solution X-ray scattering (SAXS) using synchrotron radiation.
• Analysis of Guinier plots to determine structural parameters (radius of gyration, shape).
• Proteolytic susceptibility assays using trypsin.

Main Results:

• The calmodulin-substance P complex adopts a compact globular structure dependent on Ca2+ binding.
• Ca2+ saturation reduced the radius of gyration of complexed calmodulin by 4.2 Å compared to uncomplexed calmodulin.
• Uncomplexed calmodulin exhibited a dumbbell shape, while the complexed form showed a non-dumbbell shape.
• The complex became trypsin-resistant upon binding of the first and second Ca2+ ions.

Conclusions:

• Substance P binding to calmodulin is largely complete when the C-terminal half is Ca2+-occupied.
• Significant structural rearrangements in calmodulin continue upon Ca2+ binding to the N-terminal half.
• These findings provide insights into the Ca2+-dependent structural transitions of calmodulin upon neuropeptide binding.