A new method is proposed to measure the curvature elastic modulus k c of a membrane, by observing the thermal induced fluctuations of the shape of a spherical vesicle. Observations of the liposomes were carried out under a microscospe working in the regime of Nomarski differential interference contrast. We show that the value of kc thus measured is the curvature elastic modulus of free flip-flop

The human immunodeficiency virus type 1 (HIV-1) and human T-cell leukemia virus type 1 (HTLV-1) capsid proteins (CA) display similar structures formed by two independently folded N-terminal (NTD) and Cterminal (CTD) domains. To characterize the functions harbored by the HTLV-1 CA domains in particle formation, 12 sites scattered throughout the protein were mutated. The effects of the mutations on Gag membrane binding, proteolytic processing, and virus-like particle secretion were analyzed. It appears that the NTD is the major partner of indirect or direct Gag-Gag interactions. In particular, most of the NTD mutations impaired virion morphogenesis, and no mutation located in the NTD could be fully rescued by coexpression of wild-type Gag. In contrast, the CTD seems not to be involved in Gag-Gag interactions. Nevertheless, an unknown function required for particle formation is located in the CTD. Thus, despite an overall structural similarity between the HIV-1 and HTLV-1 CA proteins, their NTDs and CTDs exhibit different functions.

First measurements of double-photon elastic scattering performed on liquids with centrosymmetric molecules yield good agreement with statistical molecular theory of nonlinear light scattering. Studies of this new scattering are shown to allow determination of the third-order molecular polarizability as well as its anisotropy, and to promise information on the structure of short-range molecular ordering and electric properties of molecules.Terhune, Maker, and Savage 1 performed the first observation of second-harmonic laser light scattering (SHLS) in liquids whose molecules lack a center of symmetry. This communication reports first measurements of double-photon elastic scattering (DPES or SHLS) by liquids consisting of molecules having a center of symmetry in their ground state. The possibility of destroying the molecular center of inversion, owing to time and space fluctuations of the molecular electric field F, led us to predict this new scattering. This is particularly predictable in the case of liquids with molecules having permanent electric quadrupole or hexadecapole moments capable of causing very strong fluctuating molecular fields in regions of shortest-range ordering in the liquid. 2 Such regions in general do not possess a center of symmetry and thus cause SHLS. This is uniquely cooperative scattering by neighboring molecules whose positions and orientations are correlated. 2 ' 3 Theoretical considerations. -Consider a laser light wave of frequency w L and propagation vector k L incident on such a liquid. The electric dipole moment M induced in the medium of volume V at frequency 2u> L is 2 ' 3is the tensor 6f third-order nonlinear polarizability induced in a molecule p at the frequency 2(x) L by the squared electric field of the light wave E UL in the presence of the molecular field ¥ ( -p) due to the AT-1 molecules surrounding molecule p with the radius vector f^.With the light beam plane polarized, propagating at velocity c along y and oscillating vertically, the intensity components of light elastically scattered at frequency 2w L with horizontal (along y) and vertical (along z) oscillations, respectively, can be written in the forms (observation along x) / y /^=#/^+i(2c^^ (2) /./"*=7^=£(2w L^^ (3 ) where I L is the incident intensity, and £=1 Q=lIn Eqs. (5) and (6) we have Ak = k 2 -2k x , where k 2 is the wave vector of the scattered second-harmonic wave and r />a = r a -"f /) is the vector connecting the centers of the two scattering molecules p and q.From Eqs.(2) and (3), we define the depolarization ratio of second-harmonic scattering byIt is obvious from the shape of the scattering factors (5) and (6) that only cooperative SHLS is present since, in the absence of molecular correlation, S 2(JJL and R 2U)L vanish. This is an important distinction with regard to SHLS by liquids consisting of molecules without centers of symmetry for which we have both a noncooperative part (isolated molecules) and a cooperative one. 2 ' 3 1295

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