Krüppel

Human homologs of Krüppel are collectively named Krüppel-like factors, a set of proteins well characterized for their role in carcinogenesis.

Krüppel expression pathway

Krüppel is expressed in the center of the embryo during the cellular blastoderm stage of development. Its expression pattern is restricted to this domain largely through interactions with the maternal effect genes Bicoid and Nanos, and fellow gap gene Hunchback and Knirps.

Bicoid maternal transcripts are deposited at the anterior end of the embryo, while Nanos maternal transcripts are located at the posterior. Hunchback mRNA transcripts are present throughout the embryo. Bicoid and Nanos both encode morphogens that have the opposite effect on Hunchback mRNA translation – Bicoid activates translation, whereas Nanos represses it. As such, Hunchback mRNA is translated so that Hunchback protein is present in the concentration gradient which decreases along the anterior – posterior axis. This Hunchback gradient indirectly results in an anterior boundary for Knirps expression. Other factors induce a posterior boundary, so that Knirps is expressed in a stripe in the posterior region of the embryo.

Hunchback and Knirps are both transcription factors that regulate Krüppel expression. High levels of Hunchback inhibit expression, whereas low levels of Hunchback activate expression. Knirps acts as a repressor to inhibit expression. This results in Krüppel being expressed in a stripe in the center of the embryo's A-P axis, where Hunchback concentration has dropped to a low enough level so that it can act as an activator, but Knirps is not yet present to inhibit. In this way the initial gradients of morphogens can lead to the establishment of a specific region within the blastoderm. It can be compared to a narrow bandwidth filter in engineering.