Volkswagen has become the largest single shareholder in Tesla rival Rivian, according to a filing by the US electric vehicle maker with the Securities and Exchange Commission (SEC).

The German carmaker now holds a 15.9% stake in Rivian, surpassing early backer Amazon, which owns about 13%, according to recent reports. Amazon uses Rivian-built electric delivery vans for parcel deliveries.

Volkswagen’s growing stake is linked to a joint venture with the US carmaker aimed at developing Rivian’s electrical architecture for VW electric models. Volkswagen has committed to invest up to $5.8 billion in the partnership.

A key feature of Rivian’s system is a so-called zonal architecture, which organizes vehicle functions by area rather than by individual systems. Traditional vehicles use separate control units for specific tasks, such as climate control.

Under Rivian’s approach, multiple functions are consolidated, allowing a single computing unit to control features such as acceleration and window operation on one side of the vehicle. The design reduces complexity and wiring length, lowering costs.

Tesla and several Chinese carmakers have also adopted similar zonal architectures.

Rivian is currently preparing to launch a lower-cost SUV model, the R2, in the US to boost sales. With its first two vehicles, Rivian’s sales remain far behind market leader Tesla.