The step-up photophobic response of the ciliated protozoan Blepharisma is proposed to be mediated by a complex of 200 kDa protein (200k) and pink-colored pigment “blepharismin” (BL) or its oxidized form “oxyblepharismin” (OxyBL).  Action spectra for the step-up photophobic response agreed with the in vivo absorption spectra of the pigment rather than with the in vitro spectra of detergent-solubilized pigment preparations.  In addition, the intensity of blepharismin fluorescence was prominently increased, when the normal structure of photosensory apparatus was disrupted by detergent solubilization.  The results suggest that in vivo configuration of pigment molecules could be quite different from that of isolated ones.  It is possible that in the living cells, a large number of blepharismin molecules are regularly arranged to form a functional photoreceptor structure, and energy may be successively transferred to neighboring pigment molecules to focus into presumed reaction centers.

 

It is suggested that the photosignal is finally transduced into H⁺ translocation from pigment vesicles to cytoplasm that produces membrane depolarization triggering generation of Ca²⁺ action potential.  Elevation in cytoplasmic H⁺ concentration evoked by protonophore suppressed the step-up photophobic response, strongly suggesting that H+ signal released from pigment vesicles to cytoplasm is closely involved in the step-up photophobic response.