Model parameter values

Model Parameter Values ^a

Parameter	Value	Reference
Componentsb
Receptors, R­T	4×105 per cell	Current work
Lyn, LT	2.8×104 per cell	Available Lyn ~0.07×RT (1)
Syk, ST	4×105 per cell	Current workc
Ligand binding
k+1	1.3×10-10 molec-1 s-1 (8×104 M-1 s-1)	Estimated binding parameters for covalently cross-linked IgE dimer (1)
k+2	2.5×10-4 molec-1 s-1	k+2RT=100 s-1 (1)
k-1, k-2	0	10-5 s-1~0 over reported time scales
Lyn association
k+L, k*+L	5×10-5 molec-1 s-1	Based on estimated equilibrium constants in Ref. (1)
k-L	20 s-1	Based on estimated equilibrium constants in Ref. (1)
k*-L	0.12 s-1	Fit to observed rate of b ITAM dephosphorylation from Ref. (2)
Syk association
k+S, k*+S	6×10-5 molec-1 s-1	Based on measured equilibrium constant at 25° C for binding of Syk tandem SH2 domains (3)
k-S, k*-S	0.13 s-1	Fit to observed rate of g ITAM dephosphorylation in Ref. (2)
Phosphorylation
pLb , pLS	30 s-1	Consistent with extensive receptor phosphorylation
p*Lb , p*LS	100 s-1	Moderate increase in Lyn kinase activity upon SH2 domain binding (4)
pLg	1 s-1	Double phosphorylation of g ITAM tyrosine, required to bind Syk, slower than single phosphorylation (5-7)
p*Lg	3 s-1	Moderate increase in Lyn kinase activity upon SH2 domain binding (4)
pSS	100 s-1	Assumed same kinase activity as recruited Lyn
p*SS	200 s-1	Moderate increase in Syk kinase activity upon phosphorylation of the activation loop (8)
Dephosphorylation
d	20 s-1	Fit to rates of ITAM dephosphorylation in Ref. (2)

^aReactions associated with these rate constants are available at here

^bCell density assumed to be 1×10⁶ cells/ml.  Cell volume assumed to be 1.4 ×10^-9 ml.

^cAssayed value of S_T is 3.4±0.4 ×10⁵ per cell, which was rounded up so that S_T=R_T.

1.             Wofsy, C., C. Torigoe, U. M. Kent, H. Metzger, and B. Goldstein. 1997. Exploiting the difference between intrinsic and extrinsic kinases: Implications for regulation of signaling by immunoreceptors. Journal of Immunology 159:5984.

2.             Mao, S. Y., and H. Metzger. 1997. Characterization of protein-tyrosine phosphatases that dephosphorylate the high affinity IgE receptor. Journal of Biological Chemistry 272:14067.

3.             Ottinger, E. A., M. C. Botfield, and S. E. Shoelson. 1998. Tandem SH2 domains confer high specificity in tyrosine kinase signaling. Journal of Biological Chemistry 273:729.

4.             Pribluda, V. S., C. Pribluda, and H. Metzger. 1994. Transphosphorylation as the mechanism by which the high-affinity receptor for IgE is phosphorylated upon aggregation. Proceedings of the National Academy of Sciences of the United States of America 91:11246.

5.             Pao, L. I., S. A. Famiglietti, and J. C. Cambier. 1998. Asymmetrical phosphorylation and function of immunoreceptor tyrosine-based activation motif tyrosines in B cell antigen receptor signal transduction. Journal of Immunology 160:3305.

6.             Pribluda, V. S., C. Pribluda, and H. Metzger. 1997. Biochemical evidence that the phosphorylated tyrosines, serines, and threonines on the aggregated high affinity receptor for IgE are in the immunoreceptor tyrosine-based activation motifs. Journal of Biological Chemistry 272:11185.

7.             Gaul, B. S., M. L. Harrison, R. L. Geahlen, R. K. Burton, and C. B. Post. 2000. Substrate recognition by the Lyn protein-tyrosine kinase: NMR structure of the immunoreceptor tyrosine-based activation motif signaling region of the B cell antigen receptor. Journal of Biological Chemistry 275:16174.

8.             Zhang, J., M. L. Billingsley, R. L. Kincaid, and R. P. Siraganian. 2000. Phosphorylation of Syk activation loop tyrosines is essential for Syk function:  An in vivo study using a specific anti-Syk activation loop phosphotyrosine antibody. Journal of Biological Chemistry 275:35442.