Figure 1

Updated working model for the AR showing a bifurcated signaling pathway downstream of cAMP/Epac. Calcium enters the cell from the extracellular milieu through the SLO-generated pores and activates sAC. Calcium also stimulates, directly or indirectly, the exchange of GDP for GTP on Rab27. Cyclic AMP synthesized by sAC activates Epac and here the signaling pathway splits into two limbs. In one of them, Epac catalyzes the exchange of GDP for GTP on Rap; in the other, cAMP/Epac indirectly activates Rab3. Rap-GTP heads a pathway that leads to acrosomal calcium mobilization (“calcium efflux” in the Figure). Rab3-GTP heads a pathway that leads to the correct assembly of the fusion machinery (“fusion competent SNAREs” in the Figure). Somewhere downstream of Rab3-GTP, there is a unidirectional (Rab3 limb → Rap1 limb) connection between both arms of the pathway. After this point, PTP1B is activated and/or recruited to the sites where it dephosphorylates NSF, derepressing its activity. Next, active, dephospho-NSF, in a complex with α-SNAP, renders SNARE proteins fusion competent. The step catalyzed by active SNAREs converges with the local increase in calcium coming from the acrosome downstream of Rap-GTP to accomplish the final steps of membrane fusion (“AR” in the Figure). Reversible and irreversible AR blockers used in this manuscript are shown in light gray. Adenophostin A (red) binds IP₃-sensitive channels and mobilizes acrosomal calcium pharmacologically. PM, plasma membrane; OAM, outer acrosomal membrane. Solid arrows mean there is one step between the terms connected, dashed arrows mean that the number of steps is either unknown or not depicted for simplicity. Modified from [12].