Presence of the complement-regulatory protein membrane cofactor protein (MCP, CD46) as a membrane-associated product...

The seminal plasma complement regulator membrane cofactor protein (MCP) was examined by sequential centrifugation and phase separation in the detergent Triton X-114. The presence of MCP components in seminal plasma depleted of the 40 kDa sperm MCP product by low speed centrifugation was confirmed. Subsequent centrifugation at 2200 g recovered a pellet containing a prominent 60 kDa and a weak 50-55 kDa MCP component. A 60 kDa MCP product remained detectable in the supernatant fraction after this centrifugation step but this was depleted by ultracentrifugation. Recovery of the seminal plasma MCP components in the pellet fraction obtained by ultracentrifugation suggested that seminal plasma MCP is membrane-associated. Seminal plasma fractions were also subjected to phase separation in Triton X-114. MCP components in both pellet and supernatant fractions partitioned to the detergent phase, confirming that seminal plasma MCP is membrane associated. The origin of these proteins was investigated by analysing MCP products in seminal plasma from vasectomized men. The 40 kDa sperm MCP protein was absent but a 60 kDa MCP component, which partitioned to the detergent phase in Triton X-114, was evident. Seminal plasma therefore contains typical membrane-associated MCP products that appear to be derived distal to the ductus deferens.