Cumulative expression of the two Lyn isoforms creates a threshold for macrophage signaling in the absence of receptor engagement.

It has been unclear whether phenotypic differences between LynA- and LynB-knockout mice, each expressing only one splice form of the Src-family kinase (SFK) Lyn, are due to isoform-specific functions or dose effects from differences in overall Lyn expression. To assess the functions of LynA and LynB, we generated a genetic series in which LynA and LynB expression levels vary independently, allowing us to test whether upregulation of one Lyn isoform could compensate for loss of the other. We used macrophages from this dose series to test signaling at steady state and in response to pharmacological pan-SFK activation. We found that LynA and LynB are equally capable of phosphorylating the positive-regulatory tyrosine kinase Syk and the negative-regulatory inositol phosphatase SHIP1, to a level determined by the cumulative expression level of LynA + LynB protein at steady state. Despite the dependence of steady-state and induced phosphorylation of upstream signaling intermediates on the level of Lyn expression, Erk and Akt signaling in response to pan-SFK activation is enabled by any Lyn expression, insensitive to further upregulation of either isoform. Thus, expression of either LynA or LynB at steady state leads to balanced potential for positive-regulatory and negative-regulatory signaling, setting a maximal response in the absence of a microbial encounter.