Energetics of Forced Thermal Ratchet

Molecular motors are known to have the high efficiency of energy transformation in the presence of thermal fluctuation. Motivated by the surprising fact, recent studies of thermal ratchet models are showing how and when work should be extracted from non-equilibrium fluctuations. One of the important finding was brought by Magnasco where he studied the temperature dependence on the fluctuation-induced current in a ratchet (multistable) system and showed that the current can generically be maximized in a finite temperature. The interesting finding has been interpreted that thermal fluctuation is not harmful for the fluctuation-induced work and even facilitates its efficiency. We show, however, this interpretation turns out to be incorrect as soon as we go into the realm of the energetics [Sekimoto,J.Phys.Soc.Jpn.66,1234-1237(1997)]: the efficiency of energy transformation is not maximized at finite temperature, even in the same system that Magnasco considered. The maximum efficiency is realized in the absence of thermal fluctuation. The result presents an open problem whether thermal fluctuation could facilitate the efficiency of energetic transformation from force-fluctuation into work.Comment: 3pages, 4sets of figure

The Carnot Cycle for Small Systems: Irreversibility and the Cost of Operations

We employ the recently developed framework of the energetics of stochastic processes (called `stochastic energetics'), to re-analyze the Carnot cycle in detail, taking account of fluctuations, without taking the thermodynamic limit. We find that both processes of connection to and disconnection from heat baths and adiabatic processes that cause distortion of the energy distribution are sources of inevitable irreversibility within the cycle. Also, the so-called null-recurrence property of the cumulative efficiency of energy conversion over many cycles and the irreversible property of isolated, purely mechanical processes under external `macroscopic' operations are discussed in relation to the impossibility of a perpetual machine, or Maxwell's demon.Comment: 11 pages with 3 figures. Resubmitted to Physical Review E. Many paragraphs have been modifie

Importin-β and the small guanosine triphosphatase Ran mediate chromosome loading of the human chromokinesin Kid

Nucleocytoplasmic transport factors mediate various cellular processes, including nuclear transport, spindle assembly, and nuclear envelope/pore formation. In this paper, we identify the chromokinesin human kinesin-like DNA binding protein (hKid) as an import cargo of the importin-α/β transport pathway and determine its nuclear localization signals (NLSs). Upon the loss of its functional NLSs, hKid exhibited reduced interactions with the mitotic chromosomes of living cells. In digitonin-permeabilized mitotic cells, hKid was bound only to the spindle and not to the chromosomes themselves. Surprisingly, hKid bound to importin-α/β was efficiently targeted to mitotic chromosomes. The addition of Ran–guanosine diphosphate and an energy source, which generates Ran–guanosine triphosphate (GTP) locally at mitotic chromosomes, enhanced the importin-β–mediated chromosome loading of hKid. Our results indicate that the association of importin-β and -α with hKid triggers the initial targeting of hKid to mitotic chromosomes and that local Ran-GTP–mediated cargo release promotes the accumulation of hKid on chromosomes. Thus, this study demonstrates a novel nucleocytoplasmic transport factor–mediated mechanism for targeting proteins to mitotic chromosomes