The elongated proteins of the spectrin family (dystrophin, alpha-actinin, and spectrin) contain tandemly repeated segments and form resilient cellular meshworks by cross-linking actin filaments. The structure of one of the repetitive segments of alpha-spectrin was determined at a 1.8 angstrom resolution. A segment consists of a three-helix bundle. A model of the interface between two tandem segments suggests that hydrophobic interactions between segments may constrain intersegment flexibility. The helix side chain interactions explain how mutations that are known to produce hemolytic anemias disrupt spectrin associations that sustain the integrity of the erythrocyte membrane.

DNA is wound tightly into phage heads in such a way that it tends to form layers concentric with the rigid protein shell. In P22 and wild-type lambda, DNA completely fills the internal volume, with a highly uniform local packing of adjacent segments; in lambda deletion mutants containing less than a full genome, the local packing distance increases correspondingly.

A system for collecting and measuring X‐ray diffraction data from protein crystals has been developed for a multiwire area detector. Computer programs run concurrently on two microcomputers, which collect and reduce detector data to integrated intensities. The self‐contained system consists of an X‐ray area detector, a rotation/oscillation camera, and two microcomputers connected by a high‐speed Ethernet network. One microcomputer is dedicated to operation of the detector, control of the camera, and storage of the raw data. The second microcomputer automatically integrates the data as they are collected and allows the user to monitor the quality of data as they are processed. The integration programs are written in Fortran 77 and have been designed to be portable. Additional programs for crystal alignment, detector and camera control, and graphics are written in the C programming language. A description of the system, some characteristics of the detector, and the results of data collection are presented.

SUMMARY1. Electrical potential differences (p.d.'s) have been measured across an in vitro preparation of rabbit gall-bladder.2. When the gall-bladder separates identical bathing solutions, the p.d. is always zero, regardless of the composition of the bathing solution. Hence the gall-bladder is symmetrical: i.e. the mucosal and serosal cell membranes have the same relative permeability coefficients.3. Osmotic water flow causes streaming potentials of up to 20 mV, of a sign indicating greater permeability to cations than to anions.4. At constant osmolarity, streaming potentials increase slightly with NaCl concentration. Streaming potentials decrease considerably with changes in osmolarity resulting from changes in NaCl concentration.5. Diffusion potentials resulting from electrolyte concentration gradients are fitted well by the constant-field equation with the relative permeability coefficients PNa = 1-00, PC1 = 0*33, PK = 2-3. These permeability coefficients are independent of osmolarity and of salt concentration.6. Relative to 0-25 mm-Ca, 5 mm-Ca reduces streaming potentials by 40 %, NaCl diffusion potentials by 62 %, and potassium diffusion potentials by 43 %.7. The aqueous channels through which water and electrolytes traverse the cell membranes of the gall-bladder contain negative fixed charges, which are blocked by Ca. The physiological significance of the charges may be to reduce chloride permeability and thereby to increase the effectiveness of the gall-bladder in concentrating bile.8. The effect of pH, and analogy with surface charges of other cells, suggest that the charges are organic acids of low pKa.