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First published online November 17, 2005
Journal of Experimental Biology 208, 4411-4418 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01906

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Increase in intracellular pH induces phosphorylation of axonemal proteins for activation of flagellar motility in starfish sperm

Ayako Nakajima¹, Masaya Morita², Akihiro Takemura², Shinji Kamimura¹ and Makoto Okuno^1,*

¹ Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan
² Sesoko Station, Tropical Biosphere Research Center, University of the Ryukyus, Sesoko, Motobu, Okinawa 905-0227, Japan

^* Author for correspondence (e-mail: cokuno{at}mail.ecc.u-tokyo.ac.jp)

Accepted 28 September 2005

Increased intracellular pH ([pH]_i) activates dynein in sea urchin and mammalian sperm and induces activation of flagellar motility. It is thought that cAMP-dependent protein phosphorylation is associated with motility activation through increasing [pH]_i, but little attention has been given to the cAMP-independent phosphorylation also induced by the [pH]_i increase. The present study demonstrates that the increase in [pH]_i in starfish sperm induces the phosphorylation of axonemal proteins and activation of flagellar motility independently of cAMP. Flagellar motility of intact sperm was activated when the [pH]_i was raised by addition of NH₄Cl. Histidine, which is known to activate motility of starfish sperm, also raised the [pH]_i during the motility activation. In addition, motility of demembranated sperm flagella was activated in a pH-dependent manner without cAMP. These results indicate that in starfish sperm it is the increase in [pH]_i that induces activation of flagellar motility. Moreover, phosphorylation of axonemal proteins (of molecular mass 25, 32 and 45 kDa) was observed during the pH-dependent and cAMP-independent motility activation of demembranated sperm. This suggests that the increase in [pH]_i regulates flagellar motility via cAMP-independent phosphorylation of axonemal proteins.

Key words: flagellar motility, regulation, axoneme, cAMP-independent, protein phosphorylation, intracellular pH, sperm, starfish

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