Inositol triphosphate receptors in sea urchin sperm

Area of Science:

• Cell Biology
• Biochemistry
• Reproductive Biology

Background:

• Inositol 1,4,5-triphosphate (Ins(1,4,5)P3) is a crucial second messenger regulating Ca2+ channels in vital cell signaling pathways.
• In sea urchin sperm, the acrosome reaction (AR) is triggered by egg investments, involving Ins(1,4,5)P3 production and opening of two Ca2+ channels.

Purpose of the Study:

• To identify and characterize a high-affinity Ins(1,4,5)P3 receptor in Strongylocentrotus purpuratus sperm.
• To investigate the potential role of this receptor in mediating Ca2+ influx during the AR.

Main Methods:

• Affinity purification of Ins(1,4,5)P3 binding component from sperm extracts.
• Characterization of binding properties including affinity, specificity, and pharmacological sensitivity.
• Immunological detection using an antibody against the type I Ins(1,4,5)P3 receptor.

Main Results:

• A 12-fold purified Ins(1,4,5)P3 binding component exhibited high affinity (KD = 261 nM) and specificity for Ins(1,4,5)P3.
• Binding was pH-dependent and inhibited by heparin.
• An antibody recognized a 240 kDa plasma membrane protein in sperm, suggesting an IP3 receptor-like molecule.

Conclusions:

• A high-affinity Ins(1,4,5)P3 receptor-like protein is present in sea urchin sperm plasma membranes.
• This protein may play a role in sustained Ca2+ uptake through the second Ca2+ channel during the acrosome reaction.