Compartmentalization of a unique ADP/ATP carrier protein SFEC (Sperm Flagellar Energy Carrier, AAC4) with glycolytic enzymes in the fibrous sheath of the human sperm flagellar principal piece

Abstract

The longest part of the sperm flagellum, the principal piece, contains the fibrous sheath, a cytoskeletal element unique to spermiogenesis. We performed mass spectrometry proteomics on isolated human fibrous sheaths identifying a unique ADP/ATP carrier protein, SFEC [AAC4], seven glycolytic enzymes previously unreported in the human sperm fibrous sheath, and sorbitol dehydrogenase. SFEC, pyruvate kinase and aldolase were co-localized by immunofluorescence to the principal piece. A homology model constructed for SFEC predicted unique residues at the entrance to the nucleotide binding pocket of SFEC that are absent in other human ADP/ATP carriers, suggesting opportunities for selective drug targeting. This study provides the first evidence of a role for an ADP/ATP carrier family member in glycolysis. The co-localization of SFEC and glycolytic enzymes in the fibrous sheath supports a growing literature that the principal piece of the flagellum is capable of generating and regulating ATP independently from mitochondrial oxidation in the mid-piece. A model is proposed that the fibrous sheath represents a highly ordered complex, analogous to the electron transport chain, in which adjacent enzymes in the glycolytic pathway are assembled to permit efficient flux of energy substrates and products with SFEC serving to mediate energy generating and energy consuming processes in the distal flagellum, possibly as a nucleotide shuttle between flagellar glycolysis, protein phosphorylation and mechanisms of motility.

FSCB, a Novel Protein Kinase A-phosphorylated Calcium-binding Protein, Is a CABYR-binding Partner Involved in Late Steps of Fibrous Sheath Biogenesis

“…Recovery of roporrin in the CABYR-FSCB complex confirms previous localizations of roporrin in the fibrous sheath (53) while pointing to specific molecular interactions between roporrin and CABYR or other interacting proteins in the complex. Confirmation of the glycolytic enzyme enolase 1 in the CABYR-FSCB complex reinforces the role of glycolysis in the fibrous sheath (40,54) while suggesting a possible role for CABYR and/or FSCB in glycolysis.…”

Evolutionarily Conserved Mammalian Adenine Nucleotide Translocase 4 Is Essential for Spermatogenesis

“…Indeed, Ant4 has distinctive N-and C-terminal regions that are conserved across mammalian species, which could potentially be adapted to a specific energy-requiring process during male meiosis or subsequent sperm function. In addition, ANT4 has been recently isolated from the fibrous sheath of the human sperm flagellar principal piece using mass spectrometry proteomics and was shown to colocalize with glycolytic enzymes (21). Ant4 may have obtained an additional function that is advantageous for mammalian fertility regarding sperm function as well.…”

“…Utilizing various approaches, we and others recently identified a novel member of the Ant family, Ant4, both in mouse and human (4,5,21). The mouse Ant4 gene was predicted to encode a 320-amino acid protein and shared amino acid sequence homology with the other mouse Ant proteins previously identified (70.1 and 69.1% overall amino acid identity to Ant1 and Ant2, respectively).…”

“…Dolce et al (4) demonstrated that human ANT4 indeed localizes to mitochondria in cells and can actively exchange ADP for ATP by an electrogenic antiport mechanism in vitro. Of interest, the Ant4 gene is expressed selectively in the testis, both in mouse and human (5,21). However, neither the exact expression pattern in testis nor the specific function of Ant4 has been determined.…”

Ultrastructure of Spermatozoa from Infertility Patients

“…The principal piece of the tail harbors glycolytic enzymes, including sperm-specific hexokinase 1, lactate dehydrogenase, and sperm-specific glyceraldehyde 3-phosphate dehydrogenase (GAPDHs) [84].…”