Related Experiment Video
Updated: Oct 21, 2026

Recording Electrical Currents across the Plasma Membrane of Mammalian Sperm Cells
Published on: February 14, 2021
Calpain-calpastatin: a novel, complete calcium-dependent protease system in human spermatozoa
F J Rojas1, M Brush, I Moretti-Rojas
1Department of Research, Worldwide Medical Corporation, Irvine, California 92618, USA.
Abstract:
Calpain, a calcium (Ca2+)-activated cysteine protease presents in several somatic mammalian cells, has been demonstrated to mediate specific Ca2+-dependent reactions including cell fusion. Because spermatozoa cells have an absolute Ca2+ requirement for penetration of oocytes, we have postulated that calpain would also be found in mammalian spermatozoa. Here we show that whole sperm homogenate and cell fractions prepared from ejaculated human spermatozoa contain calpain activity. Specific calpain inhibitors impaired this proteolytic activity. Unlike the enzyme described in somatic cells, sperm calpain was mostly particulate in nature and its activity was maximal at pH 9.0. Presence of sperm calpain was confirmed by immunoblot analysis using specific anti-calpain I and anti-calpain II antibodies. A 67 kDa calpain II protein and a 75 kDa calpain I protein were detected. Also spermatozoa contain the endogenous calpain inhibitor, calpastatin. We detected 158.8 +/- 24.5 (mean +/- SD) fmol calpastatin/mg sperm protein. Immunoblot analysis using specific antibodies showed a 68 kDa calpastatin protein located in the cytosolic fraction. This is the first demonstration that a complete calpain-calpastatin system exists in mammalian spermatozoa. Because calpain is a unique effector system for calcium-dependent processes, our data reveals a novel mechanism by which calcium exerts its regulatory functions in spermatozoa.
Related Concept Videos
Calmodulin-dependent Signaling
The Ca2+-CaM complex does not have enzymatic activity by itself. Instead, the complex binds downstream target proteins, including membrane proteins or enzymes,...
Caspases
Fertilization
Overview of Secretory Vesicles
Various proteins regulate the aggregation of molecules inside the secretory vesicles. Chromogranins...
Feedback Regulation of Calcium Concentration
Various transmembrane receptors, such as G protein-coupled receptors (GPCRs), elicit a response to extracellular signals by increasing cytosolic calcium. Activated GPCRs...
Smooth Muscle Contraction
The onset of contraction is triggered by an increase in calcium ions within the sarcoplasm, similar to the process in striated muscle. However, smooth muscles have a relatively smaller reservoir of the sarcoplasmic...

