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Diffusion Barriers in Ram and Boar Sperm Plasma Membranes: Directionality of Lipid Diffusion Across the Posterior Ring¹

^a Department of Material Food Science, Institute of Food Research, Norwich Research Park, Norwich NR4 7UA, United Kingdom ^b Department of Signalling, Babraham Institute, Cambridge CB2 4AT, United Kingdom

ABSTRACT

The plasma membrane of mammalian spermatozoa, like that of other differentiated cells, is compartmentalized into discrete regions or domains that are biochemically and functionally distinct from one another. Physical structures within the membrane, such as the posterior ring at the juncture of the sperm head and tail, have long been thought to act as diffusion barriers to help segregate important molecules required for fertilization within specific domains and to regulate migration of molecules between domains. In this investigation, we used a quantitative photobleaching technique (video-FRAP) to assess the efficacy of the posterior ring as a barrier to exchange of lipids between the postacrosomal and midpiece plasma membranes. A lipid reporter probe (1,1'-diduodecyl-3,3,3',3'-tetramethylindocarbocyanine; DiIC₁₂) was incorporated into the plasma membrane of live ram and boar spermatozoa, and the directionality of its diffusion across the posterior ring was measured by line-profile analysis. Results showed that DiIC₁₂ was able to traverse the posterior ring from the direction of the postacrosomal plasma membrane and to diffuse onto the midpiece plasma membrane. These results suggest that the posterior ring is not an immutable barrier to lipid exchange in mature spermatozoa and that there are other mechanisms for maintaining in-plane lipid asymmetry, such as differential phase behavior and interaction with the submembranous cytoskeleton.

FOOTNOTES

First decision: 25 May 2000.

¹ This work was supported by the BBSRC and a joint research fund award from the Babraham Institute and Institute of Food Research (Norwich).

² Correspondence: Roy Jones, Gamete Signalling Laboratory, Babraham Institute, Cambridge CB2 4AT, UK. FAX: 44 1223 496043; e-mail: roy.jones@bbsrc.ac.uk

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