The targeting of transmembrane proteins to lysosomes is determined by tyrosine phosphorylation and di‐leucin motifs of which there are candidates in TLR4. Tyrosine‐based motifs (YXXΦ) are found in positions 587 (YDAF), 622 (YRDF) and 707 (YLEW), whereas a di‐leucine motif ([DE]XXXL[LI]) is found in position 697 (ELYRLL), both recognized by adaptor protein (AP) complexes involved in the initial phase of endocytosis (
Bonifacino and Traub, 2003). A major additional mechanism for endocytosis and lysosomal targeting of transmembrane proteins is by the covalent attachment of one or more ubiquitins to lysines to the cytosolic domain (
Hicke, 2001;
Raiborg et al, 2003). A recent report showed that the E3 ubiquitin ligase Triad3A was involved in degradation of TLRs (except TLR2) (
Chuang and Ulevitch, 2004). Thus, we wanted to address if LPS induced ubiquitination of TLR4 and if this had any effect on endocytosis and degradation of TLR4. We found that TLR4 ubiquitination was both constitutive and enhanced by LPS in HEK293 cells expressing TLR4
YFP/MD‐2 (
Figure 4A). In monocytes, a weak increase in ubiquitination of TLR4 was observed at 1 h of LPS stimulation, followed by a reduction after 3 h (
Figure 4B). TLR4 was ubiquitinated in cells transfected with
c‐myc Ub wild type (UbRGG) but not in cells transfected with a
c‐myc nonfunctional Ub (UbR) lacking the two C‐terminal glycines required for covalent conjugation to proteins (data not shown) (
Stang et al, 2004). There was, however, no effect of UbR on uptake of LPS within the first hour of stimulation (data not shown), suggesting that LPS uptake is Ub‐independent. Ubiquitin ligases are recruited by their SH2 domains to tyrosine‐phosphorylated targets, and we found that TLR4 was indeed tyrosine phosphorylated in response to LPS (data not shown). These data suggest that although TLR4 is ubiquitinated as a response of LPS stimulation, ubiquitination is not required for the initial endocytosis of the LPS receptor complex.
Hrs is located to clathrin‐coated microdomains of the early/sorting endosomal limiting membrane where it is involved in the recognition and targeting of ubiquitinated protein cargo to the lysosomal degradation pathway by promoting translocation of the target proteins to the lumen of endosomes forming multivesicular bodies (MVB) (
Gruenberg and Stenmark, 2004). Hrs has recently also been shown to associate with phagosomes and aid in fusion with lysosomes (
Vieira et al, 2004). We investigated the involvement of Hrs in TLR4 trafficking in monocytes and in HEK293‐TLR4
YFP/MD‐2 cells transiently transfected with
c‐myc‐tagged Hrs. In monocytes Hrs constitutively associated with TLR4; however, the association increased transiently after 1 h of LPS stimulation (
Figure 4B). Control IgG did not co‐precipitate TLR4 (
Figure 4B) or Hrs (data nor shown). The TLR4 immunoprecipitated with Hrs was slightly larger and appeared more smeared than the mature form of TLR4 (
Figure 4B, middle), possibly because Hrs interacts mainly with multiple ubiquitins on the intracellular domain of TLR4. Also in HEK293 cells overexpressing
c‐myc‐Hrs, we found that TLR4 co‐precipitated with Hrs (
Figure 4C) in an LPS‐inducible manner, whereas control IgG did not (control C1,
Figure 4C). The upper band in
Figure 4C represents the c‐myc‐tagged Hrs and the lower band is the endogenous Hrs as shown by comigration with the endogenous Hrs from nontransfected HEK293 cells (control C2,
Figure 4C). Importantly, a moderate overexpression of
c‐myc Hrs (here estimated to be less than three times the endogenous level) lead to a rapid decrease in the amount of total cellular TLR4 after addition of LPS, whereas the amounts of Hrs and tubulin were unaffected (
Figure 4C). This result is in accordance with a recent paper (
Scoles et al, 2005), which demonstrated that overexpression of Hrs (e.g. less than 10 times the endogenous level) results in a rapid decrease in the EGFR levels upon stimulation with EGF. Furthermore, TLR4
CFP and Hrs
YFP were colocalized on discrete microdomains on the endosomal limiting membrane following LPS stimulation (
Figure 4D).