Materials and Methods
Experimental. Sedimentation velocity experiments were conducted
with an Optima XLI (Beckman Coulter) using an An50
Ti eight-hole rotor and with seven 400-l samples in standard
double-sector Epon centerpieces equipped with sapphire windows.
(Satisfactory results were also achieved with quartz windows.)
Interference and absorbance data were acquired simultaneously
with the absorbance scanner in the continuous mode
without averaging and with radial increments of 0.003 cm. For
the acquisition of multiple absorption signals, characteristic
wavelengths were chosen by avoiding steep slopes of the extinction
profiles of any component because of limited monochromator
accuracy and bandwidth. With this precaution, the wavelength
control was sufficiently accurate for the analysis
described. Wavelength accuracy was also found not to be problematic
when a single absorption signal was combined with
interference optical data acquisition. To accommodate possible
imperfections in the radial calibration of the absorbance and the
interference detection system, the menisci positions in the
different data sets were treated as independent parameters
optimized in the fit.
IgG, rabbit muscle aldolase, and BSA were taken from gel
filtration standard kits (catalog no. 17-0442-01, Amersham
Pharmacia). Hen egg lysozyme (HEL) was purchased from
Worthington, and D1.3 antibody was prepared as described in
ref. 29. A DNA fragment encoding the SH3 domains of phospholipase
C1 (790–851) (PLC1) was cloned in the vector
pGEX-4T-1() (Novagen) and expressed in Escherichia coli
BL21-CodonPlus(DE3)-RIL cells (Stratagene). The GST–
PLC1–SH3 fusion protein was purified by using a GSTrap FF
column (Amersham Pharmacia), digested with thrombin protease
(Amersham Pharmacia), and further purified by gel filtration
and anion exchange chromatography. The proline-rich
region of SH2-domain-containing leukocyte protein 76 (SLP-76)
(158–244) was cloned in the vector pET 28a and expressed in E.
coli BL21 cells. His-tagged SLP-76 protein was purified from a
soluble fraction with a HisTrap HP column (Amersham Pharmacia)
followed by gel filtration and anion exchange chromatography.